Tag Archives: human evolution

On Genetics and Human Behavioral Biology

Nicholas Wade, former science reporter for the New York Times has written a book, A Troublesome Inheritance, in which he argues that large-scale societal differences (e.g., the existence of capitalist democracies in the West or of paternalistic, authoritarian political systems in Asia) may be attributable to small genetic differences that were fixed at a population level through the action of natural selection since the emergence of anatomically modern humans and their subsequent dispersal from Africa. The fixation of these gene variants happened because the continents of Europe, Asia, and Africa (homes of the major “racial” groups) differed in systematic ways. David Dobbs recently reviewed it in the Sunday Review of Books, which prompted a kind of amicus brief letter-to-the-editor from over 120 population geneticists, affirming that Wade’s writing misrepresents the current science of genetics. A full list of the signatories of this letter can be found here. It is a veritable who’s who of contemporary population genetics.

As you might imagine, A Troublesome Inheritance has been quite controversial. A great deal has already been written on this book, both in formal publications and in the science (and economics) blogging ecosystem. To name just a few, Greg Laden, my old homie and fellow TF for Irv DeVore‘s famous Harvard class, Science B-29, Human Behavioral Biology, wrote a brief review here for American Scientist. Columbia statistician and political scientist, Andrew Gelman, wrote a review for Slate.com. Notre Dame professor and frequent contributor of popular work on human evolution, Agustin Fuentes, wrote a critique for Huffington Post, while UNC-C anthropology professor Jonathan Marks wrote a critique for the American Anthropological Association blog, which also appears in HuffPo.

Honestly, I think that Wade’s book is so scientifically weak and ideological (despite his protestations that science should be apolitical) that it is likely to have a very short half-life in contemporary discourse on human diversity and science more broadly. In fact, I have advocated to the editorial boards of professional societies to which I belong not to do anything special about this book since I’m confident it will be soon forgotten for its sheer scientific mediocrity. I find it interesting that the great majority of the people who like the book seem not to be scientists but comment on Wade’s “bravery” for spurning “political correctness” and the like. There are substantial parallels here to public debate over climate change or vaccination: the professional conclusions of the scientists who actually work on the topic only matter when they correspond with the social, political, or economic interests of the parties engaging in the debate. What do geneticists know about genetics anyway? So, it is with some hesitancy that I write about it, but my colleagues’ letter has reminded me of a larger beef I have with the contemporary state of human evolutionary studies. This beef boils down to the fact that most contemporary students of human evolutionary biology know next to nothing about genetics. I’ve actually encountered a number of leading figures in human behavioral biology who maintain an outright hostility toward genetics. This is a topic that my colleague Charles Roseman and I have grumbled about for a few years now. We keep threatening to do something about it, but haven’t quite gotten around to it yet. Perhaps this is a humble start…

This state of affairs is extremely problematic since genetics is the material cause (in the Aristotelean sense) or one of the mechanistic causes (in the Tinbergian sense) of much of the diversity of life. If we are going to make a scientific claim that some observed trait is the result of natural selection, we should be able to have a sense for how such a trait could evolve in the first place. The standard excuse for ignoring genetics in the adaptive analysis of a trait of interest is what Alan Grafen termed the “phenotypic gambit.” The basic idea behind the phenotypic gambit is that natural selection is strong enough to overcome whatever constraints may be acting on it. The phenotypic gambit is a powerful idea and it has yielded some productive work in behavioral ecology. I use it. However, a complete evolutionary explanation of a trait’s existence needs to consider all levels of explanation. In modern terms, and as nicely outlined a letter by Randolph Nesse, we need to answer questions about mechanism, ontogeny, phylogeny, and function. Explanations relying on the phenotypic gambit only address the functional question (i.e., fitness, or what Tinbergen called the “survival value” of the trait).

I could go on about this for a long time, so I will limit myself to three points: (1) complex traits will generally not be created by a single gene, (2) heritability and the response to selection are regularly misunderstood and misapplied, (3) we need to think about the strength of selection and the constancy of selective regimes when making statements about the adaptive evolution of specific traits.

First, we need to get over the whole one-gene thing. Among other things, the types of adaptive arguments that are made particularly for recent human behavioral innovations are simply highly implausible for single genes. There are a variety of formulae for calculating the time to fixation of advantageous alleles that depend on the particulars of the system (e.g., details about dominance, initial frequency, mutation rate). Using the approximation that the number of generations that it takes for the fixation of a highly advantageous allele with selection coefficient s is simply twice the natural logarithm of s divided by s, we can calculate the expected time to fixation for an advantageous allele. With a (very) substantial average selection coefficient of s=0.05 (think of lopping of 5% of the population each generation), the time to fixation of such a highly advantageous allele is about 120 generations generations. That’s over 3,000 years for humans. This is interesting, of course, because it makes the type of recent evolution the John Hawks or Henry Harpending have discussed more than plausible. It makes it hard to imagine how the large changes in presumably complex behavioral complexes in historical time suggested by authors such as Wade or Gregory Clark, author of Farewell to Alms (which I actually find a fascinating book), pretty implausible.

In addition to the population-genetic implausibility of single-locus evolutionary models, complex traits are polygenic, meaning that they are constructed from multiple genes, each of which typically has a small effect. Now, this doesn’t even address the issue of epigenetics, where genotype-environment interactions profoundly shape gene expression and can produce fundamentally different phenotypes in the absence of significant genetic difference, but that’s another post. In many ways, this is good news for people who study whole organisms in a naturalistic context (like human behavioral ecologists!) because it means that we can work with quantitatively-measured trait values and apply regression models to understanding their dynamics. In short, the math is easier though, admittedly, the statistics can be pretty tricky. Further good news: there are lots of people who would probably be happy to collaborate and there are plenty of training opportunities in quantitative genetics through short courses, etc.

The masterful review paper that Marc Feldman and Dick Lewontin wrote for Science in 1975 amid the controversy surrounding Arthur Jensen’s work on the genetics of intelligence, and its implications for racial educational achievement differentials, still applies. Heritability is a systematically misunderstood concept and its misuse seems to surface in policy debates approximately every twenty years. Heritability, in the strict sense, is a ratio of the total phenotypic variance that is attributable to additive genetic variance (i.e., the variance contributed by the mean effect of different alleles). Because total variance of the phenotype is in the denominator of this ratio, heritability is very much a population-specific measure. If a population has low total phenotypic variance because of a uniformly positive environment, for instance, there is more potential for a greater fraction of the total variance to be due to additive genetic variance. Think, for example, about children’s intelligence (as measured through psychometric tests) in a wealthy community with an excellent school district where most parents are college-educated and therefore have the motivation to guide their children to high scholastic achievement, the resources to supplement their children’s school instruction (e.g., hiring tutors or sending kids to enrichment programs), and the study skills and knowledge base to help their children with homework, etc. I have used this example in prior post. Given the relative uniformity of the environment, more of the variation in test scores may be attributable to additive genetic contributions and heritability would be higher than it would be in a more heterogeneous population. This is a hypothetical example, but it illustrates the rather constrained meaning of heritability and the problems associated with its application to cross-population comparisons. It is also suggestive of the problem of effect sizes of different contributions to phenotypic variance. The potential for environmental variance to swamp real additive genetic variance is quite large. What’s a better predictor of life expectancy: having a genetic predisposition to high longevity or living in a neighborhood with a high homicide rate or a endemic cholera in the drinking water supply?

Heritability essentially measures the potential response to selection, everything else being equal. The so-called Breeder’s Equation (Lush 1937) states that the change in a single quantitative phenotype (e.g., height) from one generation to the next is equal to the product of heritability and the force of selection. If there is lots of additive variability in a trait but not much selective advantage to it, the change in the mean phenotype will be small. Similarly, even if selection is very strong, the phenotype will not change much if the amount of additive variance is low. A famous, but frequently misunderstood result, known as Fisher’s Fundamental Theorem shows that the change in fitness is directly proportional to variance in fitness. This is really just a special case of the breeder’s equation, as shown in great detail in Lynch and Walsh’s textbook (and their online draft chapter 6) or in Steve Frank’s terrific book, in which the trait we care about is fitness itself. An important implication of Fisher’s theorem is that selection should deplete variance in fitness — and this makes sense if we think of selection as truncating a distribution. A corollary of Fisher’s theorem is that traits which are highly correlated with fitness should not have high heritability. Oops. Does this mean that intelligence, with its putatively very high heritabilities is not important for fitness?

Everything in the last paragraph applies to the case where we are only considering a single trait. When we consider the joint response of two or more traits to selection, we must account for correlations between traits (technically, additive genetic covariances between the traits). Sometimes these covariances will be positive; sometimes they will be negative. When the additive genetic covariance between two traits is negative, it means that selection to increase the mean of one will reduce the mean of the other. In their fundamental (1983) paper, my Imperial College colleague Russ Lande and Steven Arnold generalized the breeder’s equation to the multivariate case. The response to selection becomes a balancing act between the different force of selection, additive genetic variance, and additive genetic covariance for all the traits. Indeed, this is where constraints come from (or it’s at least one place). Suppose there are two traits (1 and 2) that share a negative covariance. Further suppose that the force of selection is positive for both but is stronger on trait 1 than it is on trait 2. Depending on the amount of genetic variance present, this could mean that the mean of trait 2 will not change or even that the mean could decrease from one generation to the next.

The work of Lande and Arnold (and many others) has spawned a huge literature on evolvability (something that Charles has moved into and that we have some nascent collaborative work on in the area of human life-history evolution). This work is very important for understanding things like the evolution of human psychology. Consider the hypothesis, popular in evolutionary psychology, that the mind is divided into a large number of specific problem-solving “modules,” each of which is the product of natural selection on the outcome of the problem-solving. How do you create so many of these “organs” in a relatively short time frame? Humans last shared a common ancestor with chimpanzees and bonobos around five million years ago and most likely human ancestors until about 1.8 million years ago seem awfully ape-like (and therefore probably not carrying around anything like the human mental toolkit in their heads). One of the key processes responsible for the creation of complex phenotypes is known as modularity (which is a bit confusing since this is also the term that evolutionary psychologists use for these mental organs!) and one of the fundamental mechanisms by which modularity is achieved is through the duplication of sets of genes responsible for existing structures. These duplicated “modules” are less constrained because of their redundancy and can evolve to form new structures. However, the fact that modules are duplicated means that they should experience substantial genetic correlation with their ancestral modules. This makes me skeptical that the diversity of hypothetical structures posited by the massive modularity hypothesis could be constructed by directional selection on each module. There is just bound to be too much correlation in the system to permit it to move in a fine-tuned way toward to phenotypic optimum for each module.

Trade-offs matter for the evolution of phenotypes. While I suspect that very few human evolutionary biologists would argue with that, I think that we generally fall short of considering the impact of trade-offs for adaptive optima. The multivariate breeders’ equation of Lande and Arnold gives us an important (though incomplete) tool for looking at these trade-offs mechanistically. A few authors have done this. The example that comes immediately to mind is Virpi Luumaa and her research group, who have done some outstanding work on the quantitative genetics of human life histories using Finnish historical records.

My third, and last (for now), point addresses the constancy of selection. This is related to the concept of the Environment of Evolutionary Adaptedness (EEA), central to the reasoning of evolutionary psychology. A few years back, I wrote quite a longish piece on this topic and its attendant problems. Note that when we use population-genetic models like the one we discussed above for the expected time to fixation of an advantageous allele, the selection coefficient s is the average value of that coefficient over time. In reality, it will fluctuate, just as the demography of the population selection is working on will vary. Variation in vital rates can have huge impacts on demographic outcomes, as my Stanford colleague Shripad Tuljapurkar has spent a career showing. It can also have enormous effects on population-genetic outcomes, which shouldn’t be too surprising since it’s the population of individuals which is governed by the demography that is passing genetic material from on generation to the next!

When I read accounts of rapid selection that rely heavily on EEA-type environments or the type of generalizations found in the second half of Wade’s book (e.g., Asians live in paternalistic, autocratic societies), my constant-environment alarm bells start to sound. I worry that we are essentializing societies. One of the all-time classic works of British Social Anthropology is Sir Edmund Leach’s groundbreaking Political systems of Highland Burma. Leach found that the social systems of northern Burma were far more fluid than anthropologists of the time typically thought was the case. One of the key results is that there was a great deal of interchange between the two major social systems in northern Burma, the Kachin and and Shan. Interestingly, the Shan, who occupied lowland valleys, practiced wet-rice agriculture, and whose social systems were highly stratified were seen by western observers as being more “civilized” than the Kachin, who occupied the hills, practiced slash-and-burn agriculture, and had much more egalitarian social relations. Leach (1954: 264) writes, “within the general Kachin-Shan complex we have, I claim, a number of unstable sub-systems. Particular communities are capable of changing from one sub-system into another.” Yale anthropologist/political scientist James Scott has extended Leach’s analysis in his recent book, The Art of Not Being Governed, and suggested that the fluid mode of social organization, where people alternate between hierarchical agrarian states, and marginal tribes depending on political, historical, and ecological vicissitudes is, in fact, the norm for the societies of Southeast Asia.

The clear implication of this work for our present discussion is that a single lineage may find some of its members struggling for existence in hierarchical states where the type of docility that Wade suggests should be advantageous would be beneficial, while descendants just a generation or two distant might find themselves in egalitarian societies where physical dominance, initiative, and energy might be more likely to determine evolutionary success. I don’t mean to imply that these generalizations regarding personality-type and evolutionary success are necessarily supported by evidence. The key here is that the social milieux of successive generations could be radically different if the models of Leach and Scott are right (and the evidence brought to bear by Scott is impressive and leads me to think that the models are right). At the very least, this will reduce the average selection differential on the putative genes for personality types that are adapted to particular socio-political environments. More likely, I suspect, it will establish quite different selective regimes — say, for behavioral flexibility through strong genotype-environment interactions!

These are some of the big issues regarding genetics and the evolution of human behavior that have been bothering me recently. I’m not sure how we go about fixing this problem, but a great place to start is by fostering more collaborations between geneticists and behavioral biologists. Of course, this would be predicated on behavioral biologists’ motivation to fully understand the origin and maintenance of phenotypes and I worry that the institutional incentives for this are not in place.

AAPA 2012 Run-Down

I am done with this year’s American Association of Physical Anthropologists annual meeting in Portland. Alas, I am not yet home as I had a scheduling snafu with Alaska Airlines yesterday and there was literally not a single seat on a flight to any airport in the Bay Area. So, I hung out in PDX for the night, where my sister-in-law is finishing up her MD/MPH at OHSU. Staying an extra night allowed me to have dinner at what is probably my favorite pizzaria on the West Coast, Bella Faccia on Alberta Ave in Northheast (Howie’s in Palo Alto is a close second). I also had a lovely breakfast of rissotto cakes and poached eggs at Petite Provance, also on Alberta. All in all, a fantastic couple days’ worth of food.
It was great to get a chance to catch up with old friends and colleagues and meet new ones. This is really what professional meetings are about. I had a chance to spend time with Charles Roseman, Rick Bribiescas, Josh Snodgrass, Nelson Ting, and Frances White. I also had very nice, if too brief, chats with Connie Mulligan, Lorena Madrigal, Larry Sugiyama, Greg Blomquist, Zarin Machanda, Melissa Emery Thompson, Cheryl Knott, and Chris Kuzawa.
I only go to the AAPAs every couple of years. Given the interdisciplinarity of my work and interests, I struggle to find a “home” professional meeting. Sometimes I feel like it’s PAA; sometimes Sunbelt; sometimes AAPA/HBA.  One thing I can say for certain is that it is not AAA, my semi-annual experience in ethnographic surreality. Such a peculiar discipline anthropology is. Part of the reason I don’t go to AAPAs all that often is that I rarely find all that much interesting there. There are a few really fantastic people working in the field but most of the talks I find stupifyingly boring. I’m just not that interested in teeth. I suppose this is true for any professional meeting, so I shouldn’t be too hard on AAPA — I’m also not that interested in contraceptive uptake, social media/online networks, or governmentality, apparently the modal topics in my competing meetings. In fact, I was pleasantly surprised by the diversity and quality of talks I saw at AAPA this year.
In my session alone, I saw really terrific and interesting talks by Steve Leigh and Connie Milligan. Steve spoke on the comparative gut microbiomes of primates and Connie presented early results on the modification of gene expression through methylation of infants born to women who experienced extreme psychosocial and physical trauma in eastern Congo. Really important stuff. It also struck me that you’d probably only see these types of talks at the AAPAs.
There were a lot of young people at this meeting — a greater fraction than I remember from past meetings.  Maybe it was the draw of hipster Portland with its great beer, great food, and general atmosphere of grooviness. Maybe there really are lots and lots of young physical anthropologists being trained these days. I must admit that I had mixed feelings about this thought as I looked out over the vast ocean of twenty-something faces in the hotel bar Saturday night. On the one hand, it’s great that people are being trained to do good work in physical anthropology. On the other hand, I worry about the ability of our discipline, which shows no signs of stopping with the charade that somehow anthropology is really akin to literary criticism, to absorb this many new Ph.D.s from (one of) the scientific wings of modern anthropology.
Two of the talks immediately before me in my session were, in fact, by young scientists and they were great. Andrew Paquette, from Northern Arizona University, gave a talk on the evolutionary history of Southeast Asian Ovalocytosis (SAO), a twenty-seven base pair deletion in the eleventh exon of the SLC4A1 gene that confers strong protection against infection with Plasmodium falciparum, the most dangerous form of malaria. Turns out this mutation, which has its geographic epicenter in Nusa Tenggara in Indonesia, is surprisingly ancient. Lots more to come from this, I’m sure. Margaux Keller, from Temple, gave a fantastic talk on finding some of the missing heritability in Parkinson’s disease. Missing heritability of complex disease phenotypes is a major topic in genetic epidemiology and Margaux and her colleagues applied Genome-Wide Complex Trait Analysis to eight cohorts of case-control studies of PD. Their results substantially increase (i.e., by a factor of 10!) the fraction of total phenotypic variance in PD explained by straight-up genome-wide association studies (GWAS). In addition to the excellent scientific content of her presentation, I was struck by the very nice and original visual aesthetic of her slides.
I spoke on my recent work on the quantiative genetics of life-history traits.  With Statistics grad student Philip Labo, I’ve been doing some pretty serious number-crunching to examine the heritabilities of and (more interestingly) genetic correlations between human life-history characters. Good results that should be seeing some more light soon (including at PAA next month!).

I am done with this year’s American Association of Physical Anthropologists annual meeting in Portland. Alas, I am not yet home as I had a scheduling snafu with Alaska Airlines yesterday and there was literally not a single seat on a flight to any airport in the Bay Area. So, I hung out in PDX for the night, where my sister-in-law is finishing up her MD/MPH at OHSU. Staying an extra night allowed me to have dinner at what is probably my favorite pizzeria on the West Coast, Bella Faccia on Alberta Ave in Northeast (Howie’s in Palo Alto is a close second). I also had a lovely breakfast of risotto cakes and poached eggs at La Petite Provence, also on Alberta. All in all, a fantastic couple days’ worth of food.

It was great to get a chance to catch up with old friends and colleagues and meet new ones. This is really what professional meetings are about. I had a chance to spend time with Charles Roseman, Rick Bribiescas, Josh Snodgrass, my EID buddy Nelson Ting, Kirstin Sterner, and Frances White. I also had very nice, if too brief, chats with Connie Mulligan, Lorena Madrigal, Larry Sugiyama, Greg Blomquist, Zarin Machanda, Melissa Emery Thompson, Cheryl Knott, Andy Marshall, and Chris Kuzawa.

I only go to the AAPAs every couple of years. Given the interdisciplinarity of my work and interests, I struggle to find a “home” professional meeting. Sometimes I feel like it’s PAA; sometimes Sunbelt; sometimes AAPA/HBA.  One thing I can say for certain is that it is not AAA, my semi-annual experience in ethnographic surreality. Such a peculiar discipline anthropology is. Part of the reason I don’t go to AAPAs all that often is that I rarely find all that much interesting there. There are a few really fantastic people working in the field, but most of the talks I find stupifyingly boring. I’m just not that interested in teeth. I suppose this is true for any professional meeting, so I shouldn’t be too hard on AAPA — I’m also not especially interested in contraceptive uptake, social media/online networks, or governmentality, apparently the modal topics in my competing meetings. In fact, I was pleasantly surprised by the diversity and quality of talks I saw at AAPA this year.

In my session alone, I saw really terrific and interesting talks by Steve Leigh and Connie Mulligan. Steve spoke on the comparative gut microbiomes of primates and Connie presented early results on the modification of gene expression through methylation of infants born to women who experienced extreme psychosocial and physical trauma in eastern Congo. Really important stuff. It also struck me that you’d probably only see these types of talks at the AAPAs.

There were a lot of young people at this meeting — a greater fraction than I remember from past meetings.  Maybe it was the draw of hipster Portland with its great beer, great food, and general atmosphere of grooviness. Maybe there really are lots and lots of young physical anthropologists being trained these days. I must admit that I had mixed feelings about this thought as I looked out over the vast river of twenty-something faces pouring into the hotel bar Saturday night. On the one hand, it’s great that people are being trained to do good work in physical anthropology. On the other hand, I worry about the ability of our discipline, which shows no signs of stopping with the charade that somehow anthropology is really akin to literary criticism, to absorb this many new Ph.D.s from (one of) the scientific wings of modern anthropology.

Two of the talks immediately before me in my session were, in fact, by young scientists and they were great. Andrew Paquette, from Northern Arizona University, gave a talk on the evolutionary history of Southeast Asian Ovalocytosis (SAO), a twenty-seven base pair deletion in the eleventh exon of the SLC4A1 gene that confers strong protection against infection with Plasmodium falciparum, the most dangerous form of malaria. Turns out this mutation, which has its geographic epicenter in Nusa Tenggara in Indonesia, is surprisingly ancient. Lots more to come from this, I’m sure. Margaux Keller, from Temple, gave a fantastic talk on finding some of the missing heritability in Parkinson’s disease. Missing heritability of complex disease phenotypes is a major topic in genetic epidemiology and Margaux and her colleagues applied Genome-Wide Complex Trait Analysis to eight cohorts of case-control studies of PD. Their results substantially increase (i.e., by a factor of 10!) the fraction of total phenotypic variance in PD explained compared to straight-up genome-wide association studies (GWAS). In addition to the excellent scientific content of her presentation, I was struck by the very nice and original visual aesthetic of her slides.

I spoke on my recent work on the quantitative genetics of life-history traits.  With Statistics grad student Philip Labo, I’ve been doing some pretty serious number-crunching to examine the heritabilities of and (more interestingly) genetic correlations between human life-history characters. Good results that should be seeing some more light soon (including at PAA next month!).

Risk Management: The Fundamental Human Adaptation

It was a conceptually dense week in class.  The first part of the week I spent talking about topics such as ecological complexity, vulnerability, adaptation, and resilience. One of the key take-home messages of this material is that uncertainty is ubiquitous in complex ecological systems.  Now, while systemic uncertainty does not mean that the world is unpatterned or erratic, it does mean that people are never sure what their foraging returns will be or whether they will come down with the flu next week or whether their neighbor will support them or turn against them in a local political fight. Because uncertainty is so ubiquitous, I see it as especially important for understanding human evolution and the capacity for adaptation. In fact, I think it’s so important a topic that I’m writing a book about it.  More on that later…

First, it’s important to distinguish two related concepts.  Uncertainty  simply means that you don’t know the outcome of a process with 100% certainty.  Outcomes are probabilistic.  Risk, on the other hand, combines both the likelihood of a negative outcome and the outcome’s severity. There could be a mildly negative outcome that has a very high probability of occurring and we would probably think that it was less risky than a more severe outcome that happened with lower probability. When a forager leaves camp for a hunt, he does not know what return he will get.  10,000 kcal? 5,000 kcal? 0 kcal? This is uncertainty.  If the hunter’s children are starving and might die if he doesn’t return with food, the outcome of returning with 0 kcal worth of food is risky as well.

Human behavioral ecology has a number of elements that distinguish it as an approach to studying human ecology and decision-making.  These features have been discussed extensively by Bruce Winterhalder and Eric Smith (1992, 2000), among others.  Included among these are: (1) the logic of natural selection, (2) hypothetico-deductive framework, (3) a piecemeal approach to understanding human behavior, (4) focus on simple (strategic) models, (5) emphasis on behavioral strategies, (6) methodological individualism.  Some others that I would add include: (7) ethological (i.e., naturalistic) data collection, (8) rich ethnographic context, (9) a focus on adaptation and behavioral flexibility in contrast to typology and progressivism.  The hypothetico-deductive framework and use of simple models (along with the logic of selection) jointly accounts for the frequent use of optimality models in behavioral ecology. Not to overdo it with the laundry lists, but optimality models also all share some common features.  These include: (1) the definition of an actor, (2) a currency and an objective function (i.e., the thing that is maximized), (3) a strategy set or set of alternative actions, and (4) a set of constraints.

For concreteness’ sake, I will focus on foraging in this discussion, though the points apply to other types of problems. When behavioral ecologists attempt to understand foraging decisions, the currency they overwhelmingly favor is the rate of energy gain. There are plenty of good reasons for this.  Check out Stephens and Krebs (1986) if you are interested. The point that I want to make here is that, ultimately, it’s not the energy itself that matters for fitness.  Rather it is what you do with it. How does a successful foraging bout increase your marginal survival probability or fertility rate? This doesn’t sound like such a big issue but it has important implications. In particular, fitness (or utility) is a function of energy return.  This means that in a variable environment, it matters how we average.  Different averages can give different answers. For example, what is the average of the square root of 10 and 2? There are two ways to do this: (1) average the two values and take the square root (i.e., take the function of the mean), and (2) take the square roots and average (i.e., take the mean of the function). The first of these is \sqrt{6}=2.45. The second is (\sqrt{10} + \sqrt{2})/2=2.29.  The function of the mean is greater than the mean of the function.  This is a result of Jensen’s inequality. The square root function is concave — it has a negative second derivative. This means that while \sqrt{x} gets bigger as x gets bigger (its first derivative is positive), the increase is incrementally smaller as x gets larger. This is commonly known as diminishing marginal utility.

Lots of things naturally show diminishing marginal gains.  Imagine foraging for berries in a blueberry bush when you’re really hungry.  When you arrive at the bush (i.e., ‘the patch’), your rate of energy gain is very high. You’re gobbling berries about as fast as you can move your hands from the bush to your mouth. But after you’ve been there a while, your rate of consumption starts to slow down.  You’re depleting the bush.  It takes longer to pick the berries because you have to reach into the interior of the bush or go around the other side or get down on the ground to get the low-hanging berries.

berryplot

Chances are, there’s going to come a point where you don’t think it’s worth the effort any more.  Maybe it’s time to find another bush; maybe you’ve got other important things to do that are incompatible with berry-picking. In his classic paper, Ric Charnov derived the conditions under which a rate-maximizing berry-picker should move on, the so-called ‘marginal value theorem’ (abandon the patch when the marginal rate of energy gain equals the mean rate for the environment). There are a number of similar marginal value solutions in ecology and evolutionary biology (they all arise from maximizing some rate or another). Two other examples: Parker derived an marginal value solution for the optimal time that a male dung fly should copulate (can’t make this stuff up). van Baalen and Sabelis derived the optimal virulence for a pathogen when the conditional probability of transmission and the contact rate between infectious and susceptible hosts trade off.

So, what does all this have to do with risk? In a word, everything.

Consider a utility curve with diminishing marginal returns.  Suppose you are at the mean, indicated by \bar{x}. Now you take a gamble.  If you’re successful, you move to x_1 and its associated utility.  However, if you fail, you move down to x_0 and its associated utility.  These two outcomes are equidistant from the mean. Because the curve is concave, the gain in utility that you get moving from \bar{x} to x_1 is much smaller than the loss you incur moving from \bar{x} to x_0.  The downside risk is much bigger than the upside gain.  This is illustrated in the following figure:

risk-aversion

When returns are variable and utility/fitness is a function of returns, we can use expected utility as a tool for understanding optimal decisions. The idea goes back to von Neumann and Morgenstern, the fathers of game theory. Expected utility has received some attention in behavioral ecology, though not as much as it deserves.  Stephens and Krebs (1986) discuss it in their definitive book on foraging theory.  Bruce Winterhalder, Flora Lu, and Bram Tucker (1999) have discussed expected utility in analyzing human foraging decisions and Bruce has also written with Paul Leslie (2002; Leslie & Winterhalder 2002) on the topic with regard to fertility decisions.  Expected utility encapsulates the very sensible idea that when faced with a choice between two options that have uncertain outcomes, choose the one with the higher average payoff. The basic idea is that the world presents variable pay-offs. Each pay-off has a utility associated with it. The best decision is the one that has the highest overall expected, or average, utility associated with it. Consider a forager deciding what type of hunt to undertake. He can go for big game but there is only a 10% chance of success. When he succeeds, he gets 10,000 kcal of energy. When he fails, he can almost always find something else on the way back home to bring to camp. 90% of the time, he will bring back 1,000 kcal.  The other option is to go for small game, which is generally much more certain endeavor. 90% of the time, he will net 2,000 units of energy.  Such small game is remarkably uniform in its payoff but sometimes (10%) the forager will get lucky and receive 3,000 kcal. We calculate the expected utility by summing the products of the probabilities and the rewards, assuming for simplicity in this case that the utility is simply the energy value (if we didn’t make this assumption, we would calculate the utilities associated with the returns first before averaging).

Big Game: 0.1*10000 + 0.9*1000 = 1900

Small Game: 0.9*2000 + 0.1*3000 = 2100

Small game is preferred because it has higher expected utility.

We can do a bit of analysis on our utility curve and show something very important about risk and expected utility. I’ll spare the mathematical details, but we can expand our utility function around the mean return using a Taylor series and then calculate expectations (i.e., average) on both sides.  The resulting expression encapsulates a lot of the theory of risk management. Let w(x) indicate the utility associated with return x (where I follow the population genetics convention that fitness is given by a w).

 \overline{w(x)} = w(\bar{x}) + \frac{1}{2} w? \mathrm{Var}(x).

Mean fitness is equal to the fitness of the mean payoff plus a term that includes the variance in x and the second derivative of the utility function.  When there is diminishing marginal utility, this will be negative.  Therefore, variance will reduce mean fitness below the fitness of the mean. When there is diminishing marginal utility, variance is bad. How bad is determined both by the magnitude of the variance but also by how curved the utility curve is.  If there is no curve, utility is a straight line and w?=0.  In that case, variance doesn’t matter.

So variance is bad for fitness.  And variance can get big. One can imagine it being quite sensible to sacrifice some mean return in exchange for a reduction in variance if this reduction outweighed the premium paid from the mean. This is exactly what we do when we purchase insurance or when a farmer sells grain futures.  This is also something that animals with parental care do.  Rather than spewing out millions of gametes in the hope that it will get lucky (e.g., like a sea urchin), animals with parental care use the energy they could spend on lots more gametes and reinvest in ensuring the survival of their offspring. This is probably also why hunter-gatherer women target reliable resources that generally have a lower mean return than other available, but risky, items.

It turns out that humans have all sorts of ways of dealing with risk, some of them embodied in our very biology.  I’m going to come up short in enumerating these because this is the central argument of my book manuscript and I don’t want to give it away (yet)! I hope to blog here in the near future about three papers that I have nearly completed that deal with risk management and the evolution of social systems, reproductive decision-making in an historical population, and foraging decisions by contemporary hunter-gatherers.  When they come out, my blog will be the first to know!

References

Charnov, E. L. 1976. Optimal foraging: The marginal value theorem. Theoretical Population Biology. 9:129-136.

Leslie, P., and B. Winterhalder. 2002. Demographic consequences of unpredictability in fertility outcomes. American Journal of Human Biology. 14 (2):168-183.

Parker, G. A., and R. A. Stuart. 1976. Animal behavior as a strategy optimizer: evolution of resource assessment strategies and optimal emigration thresholds. American Naturalist. 110 (1055-1076).

Stephens, D. W., and J. R. Krebs. 1986. Foraging theory. Princeton: Princeton University Press.

van Baalen, M., and M. W. Sabelis. 1995. The dynamics of multiple infection and the evolution of virulence. American Naturalist. 146 (6):881-910.

Winterhalder, B., and P. Leslie. 2002. Risk-sensitive fertility:The variance compensation hypothesis. Evolution and Human Behavior. 23:59-82.

Winterhalder, B., F. Lu, and B. Tucker. 1999. Risk-sensitive adaptive tactics: Models and evidence from subsistence studies in biology and anthropology. Journal of Archaeological Research. 7 (4):301-348.

Winterhalder, B., and E. A. Smith. 2000. Analyzing adaptive strategies: Human behavioral ecology at twenty-five. Evolutionary Anthropology. 9 (2):51-72.

Response to Selection

I’m done now with the first week of the Spring quarter. It was a bit challenging because I had to attend the PAA meetings in Washington, DC for the latter part of the week, but Brian Wood ably covered for me on Thursday. I thought that I would use the blog as a tool for summarizing one of the key points I want students to take away from this fist week in which we discussed evolution and natural selection.

We spent a good deal of lecture time talking about adaptation.  Specifically, we discussed how adaptation can serve as a foil to typology and essentialism. Adaptation is local and must be seen within its specific environmental and historical context. Adaptations are dynamic because environments are.

Adaptationist thinking is powerful, but can easily be overdone. This is why I also think it is essential to understand the mechanics of selection, something that I’m afraid is not often addressed in introductory evolutionary anthropology classes.  So, in the very first lecture of class, I throw some quantitative genetics (and, thus, some math) at students.  Of course, these are Stanford students, so I’m confident they can handle a little techie-ness every now and then. We specifically discuss the multivariate breeder’s equation, sometimes known as Lande‘s equation:

\Delta \mathbf{\bar{z}} = \mathbf{G \beta}

,

where \Delta \mathbf{\bar{z}} is the change in the mean fitness of a multivariate trait, \mathbf{G} is the additive genetic variance-covariance matrix, and \beta is the selection gradient on \mathbf{\bar{z}}.

In effect, \beta is a vector pointing in the direction of the optimal change in the phenotype. The matrix \mathbf{G} does two things to this gradient pushing \mathbf{\bar{z}} toward its optimum: (1) it scales the response depending on how much additive variance there is in each trait and (2) it rotates it as a function of the covariances between traits. I won’t get too much into matrix multiplication here (this is a very nice reference too). The key point is that \mathbf{G} is a square k \times k matrix (where k is the number of traits we’re looking at) the diagonal elements of which are variances and the off-diagonal elements of which, g_{ij} represent the covariances between traits i and j.   Selection requires variance. Without sufficient variance, even strong selection won’t change the phenotype much between generations.  But variance isn’t all there is to it. When the covariances are positive, there will be substantial indirect selection, and when they are negative, you have genetic constraints at work. Selection may be pointing in a particular direction, but the structure of the trade-offs could very easily mean that you can’t actually get there.

Let’s consider three quick (toy) examples.  Say we have two traits, maybe “length” and “width” (this could be something less vague and insipid: Lande (1979) looks at brain mass and body mass in a serious two-trait example). We will assume that the selection gradient is \mathbf{\beta} = \{0.5, 0.25\}?. That is, the force of selection is twice as high on length as it is on width, but it is pretty strong and positive on both. We’ll demonstrate the effect of variance and constraint in three ways:  (1) more variance in the trait under weaker selection (\mathbf{G_1}), (2) positive covariance between the two traits (\mathbf{G_2}), and (3) negative covariance between the two traits (\mathbf{G_3}).

 \mathbf{G_1} = \left( \begin{array}{cc} 0.33 & 0.00 \\ 0.00 & 0.67 \end{array} \right)

 \mathbf{G_2} = \left( \begin{array}{cc} 0.33 & 0.33 \\ 0.33 & 0.67 \end{array} \right)

 \mathbf{G_3} = \left( \begin{array}{cc} 0.33 & -0.33 \\ -0.33 & 0.67 \end{array} \right)

The figure below plots the response to selection in the three different types of genetic architecture.  The direction of selection is indicated in the grey arrow. If the variances of the two traits were equal to 1 and there were zero covariances, this is where selection would move the phenotype pair (try it). We can see that the response to selection moves toward width (the trait under weaker selection) even when covariances are zero (black arrow).  Why? Because there is more variance for width than there is for length (0.67 \times 0.25 > 0.33 \times 0.5).  This effect becomes more pronounced when there is positive covariance between the traits (blue arrow) — the selection toward width is 0.33 \times 0.5 +0.67 \times 0.25 = 0.3325. When the covariances are negative, we see something cool (red arrow).  The response to selection is small and moves (almost) entirely in the direction of length. This is because the negative covariance between length and width, when acted on by the strong selection on length, all but cancels out the positive response to selection (-0.33 \times 0.5 + 0.67 \times 0.25 = 0.0025).

selection-constraint-plot

This simple demonstration shows that the response to selection can be complex. Making an argument that some trait would be under selection is not sufficient to say that it actually evolved (or will evolve) that way.  Entirely plausible arguments for the direction of selection are made all the time in evolutionary anthropology.  Here is one from a very important paper in paleoanthropology (Lovejoy 1981: 344):

Any behavioral change that increases reproductive rate, survivorship, or both, is under selection of maximum intensity. Higher primates rely on social behavioral mechanisms to promote survivorship during all phases of the life cycle, and one could cite numerous methods by which it theoretically could be increased.  Avoidance of dietary toxins, use of more reliable food sources, and increased competence in arboreal locomotion are obvious examples. Yet these are among the many that have remained under stadong selection throughout much of the course of primate evolution, and therefore unlikely that early hominid adaptation was a product of intensified selection for adaptations almost universal to anthropoid primates.

Arguing for selection without considering trade-offs can get you into trouble.  Selection in the presence of quantitative genetic constraints (or even differential variance in the traits) can produce counter-intuitive results. (Selectionists, don’t dispair. There are ways to deal with this, but it will have to wait for another post). In the case of Lovejoy’s argument, there are good reasons to think that survivorship and reproductive rate are, indeed, strongly negatively correlated. Which is under stronger selection? Which has more additive variance? How strong are the negative covariances?

When we make selectionist or adaptationist arguments, we should always keep in the back of our minds the three questions:

  1. How strong is the force of selection?
  2. How much variance is there on which selection can act?
  3. How is the trait constrained through negative correlations with other traits?

References

Lande, R. A. 1979. Quantitative genetic analysis of multivariate evolution applied to brain: body size evolution. Evolution. 33:402-416.

Lovejoy, C. O. 1981. The origin of man. Science. 211:341-350.

Ecology, Evolution, and Human Health

Yesterday, I spent most of the day collecting content for my upcoming classes this spring and getting the course web sites together.  For the first time in a while, I will (officially) be teaching two classes in one quarter (which effectively means teaching three or four when I add the other things like lab meetings in).  The first is our graduate class on statistics in the anthropological sciences.  I taught something like this back in the old department (i.e., Anthropological Sciences) but haven’t taught it in years (though a Google search for “department of anthropological sciences stanford” turns up the syllabus for this class).  It is technically a requirement for Ph.D. students in the Ecology and Environment focus within Anthropology, so it’s about time.  It will be fun to teach again and we’re looking to use the class as a platform to develop resources for anthropologists doing statistical work (more later).

The other class that I will be teaching starting next week is Ecology, Evolution, and Human Health, a class I first taught last year. This class is meant to be an introduction to the Ecology and Environment undergraduate focus in Anthropology.  I’m actually really looking forward to teaching it again.  The course material forms the core of a book I am writing on human population biology and my attempts at improving the lectures has done wonders for my writing output of late.  We’ll see what happens when the quarter actually starts. Hopefully, between trips to Rwanda and Tanzania and moving into Arroyo House this summer, I will find time to finish it!

Back in December, when the is-anthropology-science kerfuffle was going strong, I wrote a blog post in which I suggested that if you want to feel good about the future of scientific anthropology (which, I admit, can sometimes be difficult, even for an obstinate optimist), all you need to do is look at the great work coming from the new generation of trans-disciplinary anthropologists (and other biosocial scientists).  At the time, I put together a short list of people whose work I greatly admire.  These included:

  • Craig Hadley at Emory on food security and psychological well-being
  • Amber Wutich at ASU on vulnerability, water security, and common-pool resources
  • Lance Gravlee at UF on the embodiment of racial discrimination and its manifestations in health
  • Brooke Scelza at UCLA on parental investment and childhood outcomes
  • Dan Hrushka at ASU on how cultural beliefs, norms and values interact with economic constraints to produce health outcomes
  • Crickette Sanz at Washington University on multi-ape ecology of the Goualougo Triangle, Republic of Congo
  • Herman Pontzer at CUNY on measuring daily energy expenditures in hunter-gatherers
  • Rebecca and Douglas Bird on subsistence and signaling among Martu foragers

In preparing for Anthro 31, I started to put together a list of links to people doing the kind of work we will discuss.  In a pique of obsessiveness yesterday, I greatly expanded that list.  It occurred to me that this list is somewhat orphaned in an obscure directory for a particular class I occasionally teach and that it would make sense to share it more generally.  So, here we go, copied wholesale from my class links page (though that page still contains links to books, professional societies, and other resources for students interested in human ecology, demography, health, etc.):

There are a number of excellent practicing anthropologists who maintain science blogs. Among these are Kate Clancy‘s (UIUC) Context and Variation, Daniel Lende and Greg Downey‘s Neuroanthropology, Julienne Rutherford‘s AAPA BANDIT, and Patrick Clarkin’s blog dedicated to biological anthropology, war and health, growth nutrition. Along with Rebecca Stumpf, Kate Clancy is also the director of the Laboratory for Evolutionary Endocrinology (which has its own blog) at the University of Illinois.

Upon further reflection, I think that the University of Illinois has to be a major contender for best place to study biological anthropology. Wow, they’ve got an amazing group of biological anthropologists there. Stanley Ambrose, Kate Clancy, Paul Garber, Lyle Konigsberg, Steve Leigh, Ripan Malhi, John Polk, Charles Roseman, Laura Shackelford, Rebecca Stumpf. Too many to link to directly. I don’t know all of them, but the ones I know are outstanding. Yipes! I think they may be plotting to take over the field.

Back to the blog front, you can always count on gems of anthropological, evolutionary, and political wisdom from Greg Laden as well.

Susan C. Antón (NYU) and Josh Snodgrass (Oregon) organize the Bones and Behavior Working Group, the goal of which is to foster greater synthesis across the different sub-areas of biological anthropology. Of particular interest are their standardized protocols for anthropometry.

Mario Luis Small, at the University of Chicago, has done some really outstanding work measuring how social institutions affect social capital and the impact such differences in social capital actually have for people’s well-being.

Richard Bribiescas is the author of Men: Evolutionary and Life History and is director of the Reproductive Ecology Laboratory at Yale. Yale is also now the home to Catherine Panter-Brick who also happens to be the senior editor for medical anthropology at Social Science and Medicine.

A number of excellent human biologists find their home in the Laboratory for Human Biology Research at Northwestern. This includes Bill Leonard, Thom McDade, and Chris Kuzawa. Rumor has it that alumna Elizabeth Sweet is moving back to Northwestern as well. She is doing truly innovative work integrating the rigorous analysis of biomarkers of health (and a bicultural perspective favored by the Northwestern group) and the political economy of economic and social disparities — really getting at how inequality ‘gets under the skin.’  I really look forward to seeing what comes from her future research.

Karen Kramer, in the department formerly known as (Biological) Anthropology at Harvard, is a real leader in integrating evolutionary, demographic, and economic perspectives on human reproduction and the life histories.

Patrick Clarkin at UMass, Boston has a very interesting research program employing biocultural and evolutionary models to understand the effects of war on nutrition and growth among SE Asian diaspora. UMass, Boston is also home to Colleen Nyberg who does great work on acculturation and health, the psychobiology of stress and HPA function, and growth and development.

Julienne Rutherford at the University of Illinois, Chicago School of Dentistry works on the role of the intrauterine environment on health. Of particular interest for this class is her collaborative work on understanding the epigenetic regulation of placental systems of amino acid transport as part of the Cebu Longitudinal Study in the Philippines. UIC also has a number of excellent human biologists scattered about in anthropology, including Betsy Abrams and Crystal Patil, Epidemiology (Bob Bailey) and Community Health Sciences (Nadine Peacock).

Let’s not forget our friends across The Pond. Durham may have lost Catherine Panter-Brick to Yale, but they got a number of new folks who, when combined with the veterans, make it a very appealing place to study ecological/evolutionary anthropology. Among the faculty there are my colleagues Gillian Bentley, Rebecca Sear, and Frank Marlowe, and numerous others. Rebecca does very sophisticated work in anthropological demography, while Frank is one of the leading ethnographers of contemporary hunter-gatherers (and my collaborator on our Hadza demography project).

Ruth Mace, in my opinion, does some of the best work in human behavioral ecology right now and she keeps churning out top students at UCL.

I’m looking forward to working with Mhairi Gibson at Bristol on our new project on the transmission dynamics of primate retroviruses and human-wildlife contact in Uganda. She has done excellent work on the behavioral ecology of reproduction and parental investment in Ethiopia.

I will also mention a number of excellent researchers who teach classes that are relevant to Ecology, Evolution, and Human Health:

Mark Moritz at Ohio State University has established a Hunter-Gatherer Wiki is conjunction with his course on Hunter-Gatherers. Mark came and gave a terrific talk on livestock exchanges among FulBe pastoralists at the MAPSS colloquium this year.

Mike Gurven at UCSB teaches a course on the behavioral ecology of hunter-gatherers. Mike does some of the most interesting biodemographic work out there these days.

Bruce Winterhalder at UC Davis, a founding father of human behavioral ecology, has a very interesting course on classics in cultural ecology.

Claudia Valeggia, at Penn, does great work among the Toba people of Argentina teaches a class on reproductive ecology.

Lots of good people. Lots of good work.  Surely, there is reason for optimism…

Anthropology: A Bittersweet Love Story

Rex from Savage Minds laid out a St. Valentine’s Day challenge. He asked for love letters to anthropology, in part, as a follow-up to the #aaafail fracas of December last. He notes “there is a strong chance that I’m opening the flood gates for endless cynical, bodice-ripping parodies.” But I’ll play it straight. It just so happens that the topic plays into many of the ongoing conversations I am having with friends and colleagues these days.  So, here it goes in all earnestness…
For me, anthropology is the science charged with explaining the origin and maintenance of human diversity in all its forms. To acheive this end, anthropology must be unapologetically grand in its scope.  How can we explain human diversity without documenting its full extent,  through both time and space, and across cultures? This is the thing that drew me to anthropology, the thing that really made me fall in love with it. The great story of humanity. Our great story.  Where did we come from?  What makes us human? Where does the tapestry of human diversity come from and how is it that we continually manage to resist powerful homogenizing forces and hang on to our diversity? What commonalities transcend local difference to unite all humanity? How is it that civilizations rise and fall?  And what is the fate of humanity?
This vision of anthropology relies on a simultaneous focus on difference and universality — reminiscent of Scott Fitzgerald’s famous take on true intelligence, “the ability to hold two opposed ideas in the mind at the same time, and still retain the ability to function.” It isn’t about making hyperbolic claims on flimsy or otherwise highly situated evidence. It is about relentlessly examining the commonplace with an eye to universal, the grand.
As a practitioner who came of age after the worst of the anthropology culture wars was over, what breaks my heart about the current state of our discipline is its smallness.  Anthropology has become substantially less ambitious yet so many practitioners seem utterly satisfied with this state of affairs, in large measure because we fail to engage with other disciplines. We ask trivial questions about absurdly particularistic topics.  We hesitate to make even the most unproblematic generalizations or, worse (?), make absurd generalizations on the most meagre of evidence.  We complexify rather than analyze. We theorize rather than understand. We demonize and pigeonhole our colleagues. We prefer the clever to the correct, a trait that our know-nothing discipline ironically seems to share with our hyper-rationalist colleagues in economics.
I worry for my beloved discipline’s future.  If we continue failing to connect with humanity’s big questions — if we fail to engage a broader community — we are relegated to doing poorly-funded and theoretically unsophisticated biology, literary criticism without any texts, and telling stories that no one outside our immediate circles either believes or even cares about.
For anthropology to thrive, we need to not be afraid to learn the tools that help us answer questions we want answered, rather than simply the ones that are expedient. Better still, we should have the confidence to create our own methods and develop our own theories, rather than perpetually borrowing them from our ostensibly better-endowed cognate disciplines.
One of my great intellectual heroes is Gene Hammel. Gene is an anthropologist who has published in all four subfields of anthropology; an anthropologist who gave talks to statistics departments; an anthropologist who developed new computational tools to analyze kinship and social structure long before any social scientist had a computer on his or her desk. Gene is also an anthropologist who left his anthropology department after 40 years to join a demography department because he could no longer stand the nonense of anthropology.
I wonder if this isn’t also my fate.  Was my infatuation with the immensity of anthropology simply a passionate affair of youth?  Does the mature me move on to a more sane, more stable disciplinary home? It’s a question to which I’ve given no small amount of thought recently…
…But, as I’ve said before, and I imagine I will say again, I really believe that anthropology can play a role in meeting the enormous challenges our species now faces.  Diversity is the foundation of adaptation and adpatation is always local. Understanding how different people in different places and different times solve(d) real problems provides the raw material for finding adaptive solutions to a rapidly changing world. Despite all the rhetoric one hears about living in a global world, the need for multiculturalism, blah, blah, blah, ethnocentrism and imperialist conceit are so pervasive in the contemporary academy that I seriously doubt any other discipline is likely to pick up this particular challenge. So it’s up to anthropology. However, to make this vital contribution, anthropology needs to care about the larger picture of humanity and the planet in which we are enmeshed, and anthropologists need to have the confidence to make their marks. Maintaining love after the first blush of passion has passed takes effort. Whether my discipline/lover and I are up for the joint challenge is an open question, but regardless of the outcome of couples therapy, our early relationships shape who we are and who we can become. At the very least, I will always have this vision of a grand anthropology to help guide whatever I become.

Rex from Savage Minds laid out a St. Valentine’s Day challenge. He asked for love letters to anthropology, in part, as a follow-up to the #aaafail fracas of December last. He notes “there is a strong chance that I’m opening the flood gates for endless cynical, bodice-ripping parodies.” But I’ll play it straight. It just so happens that the topic plays into many of the ongoing conversations I am having with friends and colleagues these days.  So, here it goes in all earnestness…

For me, anthropology is the science charged with explaining the origin and maintenance of human diversity in all its forms. To achieve this end, anthropology must be unapologetically grand in its scope.  How can we explain human diversity without documenting its full extent,  through both time and space, and across cultures? This is the thing that drew me to anthropology, the thing that really made me fall in love with it. The great story of humanity. Our great story.  Where did we come from?  What makes us human? Where does the tapestry of human diversity come from and how is it that we continually manage to resist powerful homogenizing forces and hang on to our diversity? What commonalities transcend local difference to unite all humanity? How is it that civilizations rise and fall?  And what is the fate of humanity?

This vision of anthropology relies on a simultaneous focus on difference and universality — reminiscent of Scott Fitzgerald’s famous take on true intelligence, “the ability to hold two opposed ideas in the mind at the same time, and still retain the ability to function.” It isn’t about making hyperbolic claims on flimsy or otherwise highly situated evidence. It is about relentlessly examining the commonplace with an eye to universal, the grand.

As a practitioner who came of age after the worst of the anthropology culture wars was over, what breaks my heart about the current state of our discipline is its smallness.  Anthropology has become substantially less ambitious yet so many practitioners seem utterly satisfied with this state of affairs, in large measure because we fail to engage with other disciplines. We ask trivial questions about absurdly particularistic topics.  We hesitate to make even the most unproblematic generalizations or, worse (?), make preposterous generalizations on the most meagre of evidence.  We complexify rather than analyze. We theorize rather than understand. We demonize and pigeonhole our colleagues. We prefer the clever to the correct, a trait that our know-nothing discipline ironically seems to share with our hyper-rationalist colleagues in economics.

I worry for my beloved discipline’s future.  If we continue failing to connect with humanity’s big questions — if we fail to engage a broader community — we are relegated to doing poorly-funded and theoretically unsophisticated biology, literary criticism without any texts, and telling stories that no one outside our immediate circles either believes or even cares about.

For anthropology to thrive, we need to not be afraid to learn the tools that help us answer questions we want answered, rather than simply the ones that are expedient. Better still, we should have the confidence to create our own methods and develop our own theories, rather than perpetually borrowing them from our ostensibly better-endowed cognate disciplines.

One of my great intellectual heroes is Gene Hammel. Gene is an anthropologist who has published in all four subfields of anthropology; an anthropologist who gave talks to statistics departments; an anthropologist who developed new computational tools to analyze kinship and social structure long before any social scientist had a computer on his or her desk. Gene is also an anthropologist who left his anthropology department after 40 years to join a demography department because he could no longer stand the nonsense of anthropology.

I wonder if this isn’t also my fate.  Was my infatuation with the immensity of anthropology simply a passionate affair of youth?  Does the mature me move on to a more sane, more stable disciplinary home? It’s a question to which I’ve given no small amount of thought recently…

…but, as I’ve said before, and I imagine I will say again, I really believe that anthropology can play a role in meeting the enormous challenges our species now faces.  Diversity is the foundation of adaptation and adaptation is always local. Understanding how different people in different places and different times solve(d) real problems provides the raw material for finding adaptive solutions to a rapidly changing world. Despite all the rhetoric one hears about living in a global world, the need for multiculturalism, blah, blah, blah, ethnocentrism and imperialist conceit are so pervasive in the contemporary academy that I seriously doubt any other discipline is likely to pick up this particular challenge. So it’s up to anthropology. However, to make this vital contribution, anthropology needs to care about the larger picture of humanity and the planet in which we are enmeshed, and anthropologists need to have the confidence to make their marks. Maintaining love after the first blush of passion has passed takes effort. Whether my discipline/lover and I are up for the joint challenge is an open question, but regardless of the outcome of couples therapy, our early relationships shape who we are and who we can become. At the very least, I will always have this vision of a grand anthropology to help guide whatever I become.

On Husserl, Hexis, and Hissy-Fits

There has been quite a brouhaha percolating through some Anthropology circles following the annual meeting of the American Anthropological Associate in New Orleans last month.  It seems that the AAA executive board, in all its wisdom, has seen fit to excise the term “science” from the Association’s long-range planning document. You can sample some of the reaction to this re-write in blog posts from anthropologi.info, Neuroanthropology, Evolution on the Beach,  AAPA BANDITInside HigherEd, and Fetishes I Don’t Get at Psychology Today. There is also a letter from AAA president, Virginia Dominguez here and you can find the full text of the planning document here. The primary concern has centered on the first paragraph of this document.  Here is that paragraph as it stood before the November meeting:

The purposes of the Association shall be to advance anthropology as the science that studies humankind in all its aspects, through archeological, biological, ethnological, and linguistic research; and to further the professional interests of American anthropologists; including the dissemination of anthropological knowledge and its use to solve human problems.

The new wording is as follows:

The purposes of the Association shall be to advance public understanding of humankind in all its aspects. This includes, but is not limited to, archaeological, biological, social, cultural, economic, political, historical, medical, visual, and linguistic anthropological research.  The Association also commits itself and to further the professional interests of anthropologists, including the dissemination of anthropological knowledge, expertise, and interpretation.

So, anthropology is no longer a science, though there are lots of rather particularistic approaches through which one can pursue anthropology that may or may not be scientific.  Apparently, the executive board has a newfound passion for public communication as well.  I guess we don’t really need an organization that promotes scholarly understanding or the production of new knowledge.  Just look where that’s gotten us!

The new wording has greatly concerned a number of parties, including the Society for Anthropological Sciences.  I am a member of this section and have never seen so much traffic on the society’s listserv.

I will admit to being somewhat dismayed by the Society’s response.  While I am not quite as tweaked by this as many, I nonetheless wrote a longish call for specific action — one that involved good old-fashioned political organizing and attempting to forge alliances both with other sections within AAA and across other scholarly societies with an interest in anthropology (e.g., AAPA, HBA, SAA, HBES).  My call was greeted with a deafening (virtual) silence and I am left to guess why.  Perhaps the membership is suspicious of the imperialist ambitions of a biological anthropologist with the taint of evolution on him?  Perhaps they’ve heard and tried it all before and were simply convinced it would not work?  Perhaps they actually like being an embattled minority and don’t really want to take action to jeopardize that status?

To what extent is the scandal a tempest in a teapot?  I honestly don’t know.  The word “science” has been taken out of the first paragraph but there is nothing inherently anti-scientific about the statement.  After all, “advancing public understanding” can be done through “archaeological, biological, social, cultural, economic, political, historical, medical, visual, and linguistic anthropological research.” Any number of these can be done through a scientific approach to understanding.

The thing that I find completely bizarre about the new wording is the exclusive focus on public understanding.  Public understanding? Really? Judging from my recent search committee and scientific review panel experience, I can only be left with the conclusion that the public must have an insatiable hunger for phenomenology.  This explains why I can never find any Husserl at Barnes and Noble — he must just be flying off the shelves!  You’d think if the goal of our flagship professional organization is really promoting public understanding, that more anthropologists would write in a manner that was generally understandable to, you know, the public.  In his distinguished lecture, the eminent archaeologist Jeremy Sabloff chastised anthropologists for their unwillingness to engage with the general public.  I could not agree with this perspective more, especially if “engaging with the public” entails engaging with colleagues from cognate disciplines, another thing that I think we do a miserable job of, in general.

I was a bit disappointed to read Alex Golub’s write-up of this issue on the Savage Minds blog.  I’m usually a big fan of both this blog and Alex’s posts more generally. However, in this case I think that Alex engages in the kind of ahistorical, totalizing stereotyping of scientific anthropologists that normally gives anthropologists the willies.  Advocates of science are characterized as close-minded automata, utterly lacking any appreciation for ambiguity, historicity, politics, or contested meaning.  For example, he writes

The fact that the model used by ‘scientific’ anthropologists has as much complexity as an average episode of WWE Smackdown — with a distinction between the evil ‘fluff-head’ cultural anthropologists and the good ‘scientific’ cultural anthropologists — should be the first sign that something fishy is going on.

Très nuanced, eh?

The statements made by many scientific anthropologists, particularly those of the generation to enter the profession in the 1960s and 1970s, need to be understood in the historical and political context of the speakers.  I think that it is simply disingenuous to claim that scientific approaches to anthropological knowledge have not become increasingly marginalized within the mainstream of anthropology over the last several decades.  One need only look at what has become to the departments that were home to the vaunted physical anthropology programs of the past to find evidence of this trend. Consider, for example, the University of Chicago, the University of California Berkeley or Columbia University.  And this is just biological anthropology; it does not account for the loss of scientific social and cultural anthropologists (think Gene Hammel or Roy D’Andrade) in elite, Ph.D.-granting programs. The reasons for the marginalization of scientific approaches to anthropology are complex and do not fit neatly into the simplistic narrative of “objective, scientific anthropology … under assault from interpretivists like Clifford Geertz who do not believe in truth.” No doubt, part of the problem is simply the compartmentalization of knowledge.  As scholars become increasingly specialized, it becomes more and more difficult to be both scientist and humanist.  Increasingly, hiring decisions are zero-sum games. The gain of a scientist represents the loss of a humanist and vice-versa. Gone is Eric Wolf’s conception of Anthropology as “both the most scientific of the humanities and the most humanist of the sciences.”

The key is that the declining importance of science in the elite anthropology departments has led to a feeling of embattlement — that is almost certainly counter-productive most of the time — among the remaining scientific anthropologists. Another consequence is that the decline of the place of science within anthropological discourse selects for personalities who thrive on embattlement, so that the reproduction of the field is relatively enriched with young scholars who see no point to professional or intellectual engagement. And so it gets more and more difficult to integrate.  This is the lens through which I view much of the public complaining about the recent actions of the AAA executive board. However, as my colleague Rebecca Bird noted, those of us who still see a place for science in anthropology need to move beyond reactionary statements.  We need to be proactive and positive.

The academy is changing. This can be seen in the increasing number of cross-cutting requests-for-proposals from funding agencies such as NSF (e.g., HSD, EID, CHNS) or NIH and the wholesale re-organization of many research universities (ASU is only the most extreme case; the ascendency of interdisciplinary centers such as the Woods Institute for the Environment or the Freeman-Spogli Institute for International Studies at Stanford is a more common manifestation of this trend; the Columbia Earth Institute also comes to mind).  In an academy that increasingly values transdisciplinarity and integration of knowledge, I think that anthropologists have an enormous comparative advantage — if we could just get over ourselves.  As I wrote in my 2009 Anthropology News piece:

Four-field anthropology is a biosocial discipline that integrates information from all levels of biological and social organization. To understand human behavior, the four-field anthropologist considers genetics and physiology; the history of the human lineage; historical, cultural and social processes; the dynamics of face-to-face interactions; and global political economy. Each of these individual areas is studied by other disciplines, but no other field provides the grounding in all, along with the specific mandate to understand the scope of human diversity. The anthropologist stands in a unique position to serve as the fulcrum upon which the quality of an interdisciplinary research team balances. Revitalizing the four-subfield approach to anthropological training could move anthropology from the margins of the interdisciplinary, research-based academy of the near future to the core.

I have no interest in disparaging forms of knowledge or excluding particular types of scholars from any social movement, but I think that scientific anthropologists have a particularly important role to play in such a revitalization, if for no other reason than they (presumably) care about more of these levels of organization.  Maybe such scholars could even communicate the subtlety and richness of ethnographic experience that our more humanistic colleagues so value if we could just get beyond the name-calling.

I may be dismissed as being naively optimistic by the old guard of scientific anthropologists (hypothesis 2, above), but I think that I have good reasons to be optimistic about the future of anthropology, despite the many challenges. This optimism stems from the work of individual anthropologists.  I’ll do a quick shout-out to a number of people who I think are doing particularly good work, integrating different anthropological perspectives, and communicating with a broader audience.  This is a very personal and idiosyncratic list — these scholars are people I’ve encountered recently or whose work has been brought to my attention of late. They tend to be focused on questions of health and human-environment interactions, naturally, since these are the issues that organize my research.

If you want to feel good about the future of a scientific anthropology that is simultaneously integrated into contemporary anthropology and communicates with a broader scientific and policy audience (and is generally great and transformative — that key NSF buzz word), check out the work of:

  • Craig Hadley at Emory on food security and psychological well-being
  • Amber Wutich at ASU on vulnerability, water security, and common-pool resources
  • Lance Gravlee at UF on the embodiment of racial discrimination and its manifestations in health
  • Brooke Scelza at UCLA on parental investment and childhood outcomes
  • Dan Hrushka at ASU on how cultural beliefs, norms and values interact with economic constraints to produce health outcomes
  • Crickette Sanz at Washington University on multi-ape ecology of the Goualougo Triangle, Republic of Congo
  • Herman Pontzer at CUNY on measuring daily energy expenditures in hunter-gatherers
  • Rebecca and Douglas Bird on subsistence and signaling among Martu foragers

This list could go on. I won’t even mention the amazing anthropology post-docs, Siobhan MattisonSean Downey, and Brian Wood, with whom I have been so lucky to interact this academic year.

I have plenty more to say on this — particularly how the portrayal of politics and political agendas enters the discourse — but I have final exams to grade!

Best Simile Ever?

Matt Ridley pens a hilarious simile in his great book, Nature Via Nurture (published as The Agile Gene in the United States) that I think you might actually need to be an evolutionary anthropologist to fully appreciate.  And I quote:

Just as sex enabled mammals to combine two great inventions — lactation and the placenta — so trade enabled early people to combine draft animals and wheels to better effect. (Ridley 2003: 228)

Just like it.  Awesome!

Most Cited Papers in Current Anthropology

A friend sent me a link the other day to the top 20 most cited articles in the journal, Current Anthropology. Much to my delight, I found that a paper that I co-authored is the #7 all-time citation leader and a paper co-authored by my Stanford colleague Rebecca Bird is the #19. As I walked over to Coupa café this morning to get coffee, I realized that I also made a small contribution to the #1 on this list, Leslie Aiello and Peter Wheeler’s paper on the Expensive Tissue Hypothesis.  At the time the manuscript was first circulated, I was a graduate student obsessed with brains, energetics, and scaling in human evolution. My advisor, Richard Wrangham, was asked to comment on the manuscript and he asked me if, given my obsessions, I might have something to say. Needless to say, I did. Having just read our comment, I think it stands pretty well (if I do say so): (1) basal metabolic rate (BMR) is not really a constraint and (2) what are the implications for allometric scaling of different organs with respect to body mass?  Most of the expensive organs scale isometrically (that is, with a scaling exponent of one) but the brain, of course, is a big exception. It scales with an exponent closer to 3/4. Because guts and brains scale differently with increasing body mass, perhaps larger brains could be maintained by dietary compensation?

My colleague Herman Pontzer has some very interesting things to say about energetics and constraints and I’m really looking forward to some forthcoming work of his on this topic.  In a paper in PNAS, he recently showed that, contrary to the expectations of a naïve trade-off model, mammals with larger home ranges actually have greater lifetime fertility and greater total offspring mass.  We have a lot to learn about trade-offs, both physiological and economic, and their role in shaping human behavior and life histories.

More on Buller and Evolutionary Psychology

This is an ongoing series of meditations on evolutionary psychology inspired by my recent reading of David Buller’s piece in Scientific American.  I have been thinking quite a bit in the last year about the relationship between evolutionary psychology, human behavioral ecology, and evolutionary genetics, and maybe these ruminations will help me get my thoughts clear on these difficult topics.  Caveat utilitor: these are not fully formed ideas but the blog is a useful device for organizing my sketches.

I found an interesting  critique of Philosopher of Science and evolutionary psychology critic David Buller‘s book, Adapting Minds. Edouard Machery and H. Clark Barrett wrote an extended, critical review of Buller’s 2005 book in the journal Philosophy of Science.

I must admit that I find myself torn on some of these debates. I am sympathetic to many of the criticisms voiced by Buller, but think that some of the rebuttals are quite compelling as well. For example, Buller is highly critical of work on child homicide by Martin Daly and Margo Wilson of McMaster University.  Daly and Wilson, in a series of famous studies, suggest that child homicide (a rare event) is much more likely to be perpetrated by step-parents (including boyfriends).  The explanation for why this might be relates to the existence of an anti-cuckoldry mechanism in men’s brains. Given the enormous obligate investment — generally on the part of two parents — entailed in the successful recruitment of human offspring, cuckoldry represents a potentially enormous fitness cost for human men.

In one study of child homicides in Canada between 1974 and 1990, Daly and Wilson calculated a risk-ratio that child homicides are perpetrated by step-parents vs. (putative) biological parents of 123.7.  Buller suggests that such results might simply arise because of ascertainment bias in the reporting of child homicide.  Specifically, he suggests that the cause of death listed on a child’s death certificate is far less likely to be homicide if the act was perpetrated by a biological parent. In support of this argument, he cites a paper by Crume et al. (2002) which compared cause-of-death as listed on the death certificate with the cause determined by a interagency multidisciplinary child fatality review team.  This team reviewed child deaths in the state of Colorado and found that a substantial number of likely homicides were not reported as such.  They were then able to investigate which attributes of (alleged) perpetrators made ascertainment more or less likely.  They found that homicides committed by non-relatives (including boyfriends) were 8.41 times more likely to be recorded as such than were those committed by parents. Of 152 death at the hands of parents only 65 were correctly ascertained while 87 were not.  For the 51 deaths attributable to non-relatives, 44 were correctly ascertained while seven were not.  This yields an odds ratio of (44*87)/(65*7)=8.41 that non-relatives will be correctly ascertained compared to parents (the OR changes to 8.71 following multivariate adjustment — it is this number that is discussed in the various papers). This seems pretty damning (and suggests there are major problems detecting fatal violence against children).  However, one point from this paper that Buller does not note in his critique (at least his 2005 paper in Trends in Cognitive Sciences) is that the odds of ascertainment for non-parent relatives — including step-parents — is not significantly different from unity. That is, the group that includes step-parents is as likely to be ascertained as biological parents.  My understanding is that Daly and Wilson’s analysis applies to step-parents as well as boyfriends.  The theory certainly predicts this.

My sense is that Buller is reaching a little too far in this critique. While I would hardly consider myself an expert on the topic, I have always thought quite highly of Daly and Wilson’s demographic work on homicide.  One of my students is currently relying heavily on their Chicago mortality study published in BMJ.  That is something I do have some expertise in and I think it is excellent.  What I want to know is this: what is the counterfactual to the Daly & Wilson work?  How many child deaths would need to be re-classified in order to have ascertainment bias be sufficient to account for their observed differences?  Daly & Wilson (2007) do just this sort of counterfactual calculation.  They assume that step-fathers are always caught, whereas biological fathers are never caught.  According to their calculation, such a scenario would imply that there were 500 unaccounted-for paternal murders to yield the observed rates.  This is where the problem comes in.  There simply aren’t 500 deaths each year to children under five in Canada in that period that aren’t due to congenital defects or infectious disease.  Mortality among the young is rare in developed countries. Clearly, not all of the effect that Daly & Wilson report can be attributed to ascertainment bias.  There seems to be some there there.

I think that this over-reaching is a shame.  The critiques that Buller levels in his recent Scientific American piece are serious and deserve to be taken seriously. Here, I specifically mean the idea that an analysis of the Pleistocene will yield significant clues for understanding the design of the human mind and that evolutionary psychology will be much use in helping us understand unique and universal human traits.  The tone of this debate (on both sides) seems to preclude serious consideration of these important concerns.

As I mentioned in my previous post, I find the latter problem particularly troubling because it suggests that there are some things we can never know about human evolution in a scientific way.  Depending on the question, one possible solution to this problem is something Marc Hauser used to talk about in Science B-29 at Harvard.  The problem was how to use evolutionary tools to explain the unique phenomenon of human language.  While human language is clearly a unique, derived trait — and therefore in a difficult position with respect to scientific explanation — there are features of human language (e.g., those described by Hockett in his design features of human language) that are shared across multiple species, making them amenable to the comparative method.  If we limit ourselves to specific autapomorphies — as Buller apparently wants us to when it comes to Human Nature — then we are sunk.  If we can find features of our cognition that are shared across species and look, as Darwin first suggested, at convergent solutions to similar problems across species, then we may have some hope of understanding the unique whole of human cognition. Of course, we can’t do this for cognitive features that have arisen since the Pleistocene because we only have one remnant of the hominin clade left (us).

Regarding our ability to understand the design of the human brain based on our knowledge of the environment of Pleistocene hunter-gatherers, Machery and Barrett (2006: 236) write that Pleistocene hominins experienced a “reduction in sexual dimorphism in body size due to increased pair bonding and male investment in offspring and corresponding reduction in male-male competition.” While I happen to agree with this point (and have two new papers either submitted or in prep elaborating my take on this particular phenomenon), it is, in fact, conjectural.  There is nothing to stop us from forming hypotheses about the mechanisms or functional consequences of human behavior that result from this conjecture, and there might be substantial value in doing so.  Nonetheless, I think it’s important to note that it is hardly certain that the cause of the reduction in sexual dimorphism among Pleistocene hominins (something we are pretty sure of) was pair bonding.  I’m afraid to say that I am not the least bit confident that we will ever know this for certain.

Why do we think that Pleistocene hominins were “pair bonded”?  We know that sexual size dimorphism is a correlate of mating system.  Polygynous mammals tend to be sexually dimorphic.  The more polygynous, the more dimorphic.  Presumably, this arises through intra-male mating competition, where size matters for the outcome of agonistic encounters.  As detailed in our 1999 paper, the best paleontological evidence we have suggests that there was a substantial reduction in both sexual size dimorphism and dimorphism in canine teeth (another strong correlate of polygyny among Primates) with the emergence of the genus Homo.  This reduction in sexual dimorphism is attributed by many authors, ourselves included, as a signal of a change in mating system toward increased monogamy.  Does monogamy necessarily mean pair-bonding?  Not necessarily. (again, I do think it’s true in this case and hopefully, I will finish the paper in which I discuss the details of this argument soon)  There is also the issue that humans are not much different in terms of sexual size dimorphism from chimpanzees, whose mating system is completely promiscuous.  Our teeth may rescue us here.  Chimpanzees are quite sexually dimorphic in their canine teeth.  But how do you weigh the importance of canines as a weapon in a species that makes tools, including weapons that allow it to kill from a distance?

My point here is that there is a good deal of uncertainty about basic aspects of Pleistocene hominin behavior.  This uncertainty is unlikely to ever be completely resolved.  As a result, I’m not convinced that looking for clues about human behavior and the design of the human brain in the behavior of Pleistocene hominins is necessarily the most efficient of productive avenue for understanding our psychology. I don’t take the absolutist position that Buller seems to take that there is nothing to be learned about the present by studying the deep past (i.e., it is more than “pure guesswork”).  I like the iterative approach of working between hypothesis generation and empirical test that Machery and Barrett describe and think that it sounds an awful lot like the process that most scientists employ in their work and it sounds like the way individuals adapt to dynamic environments.

I’ll end this ramble with a question: Do you have to be an evolutionary psychologist to believe in Human Nature?  Buller seems to think so and to think that it’s a bad idea.  I don’t think of myself as an evolutionary psychologist, but I do think there is such a thing as Human Nature.  I am struck by the fact that despite the dizzying array of cultural diversity that is manifested by our species, a smile is a smile, embarrassment is embarrassment, and a look of consternation is a look of consternation.  We might find different things amusing, mortifying, or distressing but pretty much people everywhere experience these emotions and, because of our theory of mind, recognize them in others.  The work of Eckman, Eibl-Eibesfeldt, and Fernald, to name a few, is pretty compelling in this regard.  Do we have a cheater-detection module that was engineered in the Pleistocene?  Maybe.  Honestly, I don’t care that much, but I do think that denying the existence of Human Nature is done at our collective peril.

References

Buller, D. J. (2005). Evolutionary psychology: the emperor’s new paradigm. Trends in Cognitive Sciences, 9(6), 277-283.

Crume, T. L., DiGuiseppi, C., Byers, T., Sirotnak, A. P., & Garrett, C. J. (2002). Underascertainment of Child Maltreatment Fatalities by Death Certificates, 1990–1998. Pediatrics, 110(2), 1-6.

Daly M, Wilson M (2007) Is the “Cinderella effect” controversial? A case study of evolution-minded research and critiques thereof. In C Crawford & D Krebs, eds., Foundations of evolutionary psychology. Mahwah NJ: Erlbaum.

Machery, E., & Barrett, H. C. (2006). Essay Review: Debunking Adapting Minds. Philosophy of Science, 73, 232-246.

Wilson, M., & Daly, M. (1997). Life expectancy, economic inequality, homicide, and reproductive timing in Chicago neighbourhoods. British Medical Journal, 314(7089), 1271-1274.