Saturday, August 27, 2011

Was evolutionary psychology inevitable?

Cover of Time, August 28, 1995. Evolutionary psychology beat out its rivals in the race to win public acceptance.

During the 1990s, evolutionary psychology overtook and replaced sociobiology. Its success was total, much like that of many paradigms we now accept as normal science. Did it succeed for the same reason? Did it better fit the data?

In some ways evolutionary psychology was better than its rivals and in other ways worse. Its superiority tended to be more organizational, even political. In short, it became the only game in town for study of human behavior from an evolutionary perspective. The alternatives—sociobiology and gene-culture co-evolution—had much less to offer in terms of research funding, career opportunities, or just an untroubled academic life.

Early years of evolutionary psychology

At first, the term ‘evolutionary psychology’ simply referred to psychologists and psychiatrists who shared an interest in evolutionary theory. Such people began to come together in the late 1980s:

In mid-1987, an initial list of people using modern evolutionary theory to address problems in psychology and psychiatry was compiled and distributed. The respondents suggested additional names that expanded the list to over 200 researchers. Many people expressed surprise that there were so many others working in the area. Many more suggested that it was time to organize a forum for the exchange of ideas and research findings in the area. (Evolution, Psychology and Psychiatry 1988)

The first forum was held at the 1988 Evolution and Human Behavior Meeting in Ann Arbor, Michigan. The meeting had two purposes:

- To facilitate the exchange of ideas and findings among researchers in the area of Evolution, Psychology and Psychiatry. The developing new basic science will be emphasized.

- To consider plans for initiating an organization that will promote research and scholarly exchange in the area of Evolution, Psychology and Psychiatry.

The proceedings only hinted at evolutionary psychology’s eventual divorce from sociobiology. Some presenters talked about a mismatch between current behavior and earlier contexts of adaptation, but these contexts were not placed over a million years ago in the Pleistocene.

It was decided to have more meetings and to hold them annually. The 1989 meeting, now called the Human Behavior and Evolution Conference, had a similar mix of ideas. Presenters were now talking more about ‘mismatch’ and ‘mental mechanisms’:

Debate about the relevance of mental mechanisms, rather than current human behavior, to the study of adaptation continued from other conferences in the recent past. […] Turke pointed out that studies examining current reproductive success could reveal the nature of mental mechanisms by comparing contexts in which humans do or do not behave adaptively. […] The abstract models presented by evolutionary psychologists (first by Cosmides and Tooby), which are somewhat like descriptive structural equation models, seem often to be misunderstood to present physiological structures, again making mental mechanisms appear to promote invariant behavior. More sophisticated basic theory is necessary to determine whether current behavior reflects adapted strategies. (HBES, 1990)

Leda Cosmides and John Tooby were already arguing that the genetic determinants of human behavior had assumed their current form long before the emergence of Homo sapiens, but this view was still a minority one. Other presenters were affirming the possibility of much more recent evolution:

Cultural versus natural selection was also the major topic of a roundtable on the final morning, lead by William Hamilton, George Williams, Irenaus Eibl-Eibesfelt, Edward Wilson, Irven DeVore, and Richard Dawkins. Much of the discussion focused on the unit of selection and the selective process. Dawkins stressed defining a specific replicating unit, which must be genes or memes. Williams proposed using epidemiological, disease–transmission models for the spread of cultural traits rather than using genetic models. Wilson stressed the importance of co-evolutionary theory, which models the interaction of genes and culture in epigenetic processes. (HBES, 1990)

Evolutionary psychology was nonetheless en route to becoming a paradigm distinct from sociobiology, with the Pleistocene EEA as the key difference. In 1989, this theme inspired two articles in Ethology and Sociobiology. Don Symons wrote:

[…] a well-formed description of an adaptation must consist solely of words for things, events, relations, and so forth that existed in the EEA, which, in the case of human beings, means the Pleistocene world of nomadic foragers. The specific environmental features to which Tibetan polyandry is adapted, according to Crook and Crook, include agricultural estates, animal husbandry, primogeniture, monasticism, aristocrats, landlords, governments, and taxation. Because none of these things existed in the human EEA, Crook and Crook’s account contains not a single well formed description of a Darwinian adaptation […] (Symons, 1989, pp. 138-139)

John Tooby and Leda Cosmides argued along similar lines:

It is no more plausible to believe that whole new mental organs could evolve since the Pleistocene—i.e., over historical time—than it is to believe that whole new physical organs such as eyes would evolve over brief spans. It is easily imaginable that such things as the population mean retinal sensitivity might modestly shift over historical time, and similarly minor modifications might have been made in various psychological mechanisms. However, major and intricate changes in innately specified information-processing procedures present in human psychological mechanisms do not seem likely to have taken place over brief spans of historical time. (Tooby & Cosmides, 1989, p. 34)

Here, Tooby and Cosmides assert that ‘whole new mental organs’ could not have arisen over the past million years. They then conclude that ‘major changes’ to mental organs are unlikely. This conclusion hardly follows from their initial assertion, which itself is doubtful. New organs have arisen over shorter time spans, specifically through duplication of an existing organ and rapid modification of the ‘spare.’

As a result of these articles and succeeding ones, the Pleistocene EEA came to define evolutionary psychology. The latter ceased to mean ‘sociobiology for psychologists’ and took on a narrower, more paradigmatic meaning.

Gene-culture co-evolution: a competing paradigm

At the turn of the 1990s, it was still far from clear that evolutionary psychology would take over from sociobiology. Gene-culture co-evolution seemed to hold a stronger position.

This other paradigm began in a cultural evolution class that L.L. Cavalli-Sforza taught in 1978-79 at Stanford to Robert Boyd and Peter Richerson (Stone & Lurquin 2005, p. 108). The Italian geneticist argued that the natural environment has not been the main driving force of human evolution. Instead, this role has largely fallen to the cultural environment, such as oral and written language, social organization, technology, means of subsistence, and so forth. There has thus been a feedback loop: we humans have created different cultural environments, which in turn have subjected us to different pressures of natural selection. Our creations have become our creators. This co-evolution fascinated Cavalli-Sforza, and its study now occupied his thoughts.

But others were thinking along the same lines, like the anthropologist Claude Lévi-Strauss in 1979:

[…] as anthropologists, the aspects of the question that will always appeal to us will be much less the genetic determination of culture or cultures than the cultural determination of genetics. […]

The selection pressure of culture—the fact that it favors certain types of individuals rather than others through its forms of organization, its ideas of morality, and its aesthetic values—can do infinitely more to alter a gene pool than the gene pool can do to shape a culture, all the more so because a culture’s rate of change can certainly be much faster than the phenomena of genetic drift.
(Lévi-Strauss, 1979, p. 24-25)

The 1980s saw growing interest from other students of human evolution. Robert Boyd and Peter Richerson were among the first, followed by Pierre van den Berghe, Charles Lumsden, E.O. Wilson, and R.D. Alexander. The new paradigm was summed up by Pierre van den Berghe:

[…] the causality between genes and culture is reciprocal […] culture, though clearly possessing some emergent and irreducible properties (notably in its mode of transmission), is itself subject to a wide set of developmental parameters because it comes out of a brain with a definite bio-chemical structure […] the human mind, despite its undoubted plasticity, is not a tabula rasa. It is predisposed to learn certain things with ease, and others with difficulty or not at all. According to this viewpoint, the programmed learning biases of the human brain result in a non-random distribution of cultural products […] Moreover, since culture has consequences for the survival and reproduction of flesh and blood organisms, natural selection must exert some influence on cultural evolution (van den Berghe & Frost, 1986)

Work on gene-culture co-evolution initially involved trawling through the ethnographic literature. In the mid-1980s, however, a major project for field research was launched, specifically among the Inuit of northern Canada.

Its leader? Cavalli-Sforza. The project was organized in 1986 with professors from Queen’s University and Université Laval. The aim was to determine whether natural selection favors different mental toolkits in hunting and gathering societies versus agricultural societies. As one project member, John Berry, a psychologist at Queen’s University, later explained:

Hunters, by this way of thinking, require good visual acuity, keen disembedding skills and a well-developed sense of spatial orientation. To hunt successfully, the hunter must be able to discern the object of the quest (which is often embedded in a complex visual landscape), then disembed the object, and finally return to home base. In contrast, agriculturalists need not develop these particular skills, but rather they need to invest in other areas of development, such as conservation (in both the economic and the Piagetian senses) and close social interactions. (Berry 2008, p. 3)

According to an unpublished report, Cavalli-Sforza wished to trace the origins of Inuit artistic talent by estimating the relative contributions of genetic endowment and socio-cultural learning:

One of the most remarkable phenomena in the contemporary Canadian Arctic is the presence of highly-acclaimed art forms — carving in stone and ivory, and printing on paper. The question we ask is: how can we account for the wide-spread distribution of such talent in a small dispersed population?

[…] Is it possible that artistic talent is transmitted culturally (from parents to offspring, from others in society to the artist, and from peers to artist)? How can we assess these types of transmission?

Is it possible that artistic talent is transmitted genetically (from parents to offspring)? How can we assess such transmission?
(Berry & Cavalli-Sforza 1986, p. 2)

The approach would be to study specific Inuit artists and then their biological and non-biological relatives. The high rate of adoption among Inuit (between 15 and 30%) would provide a means to distinguish cultural inheritance from genetic inheritance in the transmission of artistic talent. Related mental traits would also be investigated. “Given enough information one can hope to separately estimate two quantities, called respectively cultural and genetic heritability” (Berry & Cavalli-Sforza 1986, p. 5)

The project fell through. At Laval, we assumed something went wrong with the funding. At Queen’s, the story was that Cavalli-Sforza had quit because of illness. Yet his biography makes no mention of illness during this period, the only bouts of ill health being an operation for bladder cancer in 1976 and a heart attack in 1991 (Stone & Lurquin, 2005, pp. 98, 160). In any case, ill health would have been a reason for postponing the project, not canceling it.

This project has left no traces in any of Cavalli-Sforza’s publications—books, journal articles, conference proceedings, or poster sessions. The paper trail amounts to one unpublished report (Berry & Cavalli-Sforza, 1986). Just as mute are his published writings on gene-culture co-evolution. Examples are confined to the usual suspects: lactose tolerance in cattle-raising societies, and malaria resistance among tropical agriculturalists (Cavalli-Sforza & Cavalli-Sforza 2008, p. 264). There is no hint that natural selection might have favored different mental traits in different cultural environments.


With Cavalli-Sforza gone, the field of gene-culture co-evolution lost momentum. It had big names, but few of them could put boots on the ground, i.e., do fieldwork. Nor could any of them focus on this area of research. As big names, they typically had their fingers in many pies.

There may have been another reason in the case of Cavalli-Sforza. He was vulnerable to blackmail because of his wartime anthrax research … in Berlin. Nothing terrible came of that laboratory work, but it had the potential to derail his career. At the very least, it was a stain on his curriculum vitae. (1)

Evolutionary psychology thus beat out its rivals by default. The term ‘sociobiology’ was becoming a mark of Cain even among people interested in human behavior and evolution. In 1997, it was literally voted out of existence. Gene-culture co-evolution continued to attract interest but only as a sideline.

With its rivals growing ever weaker, evolutionary psychology gained strength as the millennium drew nearer. It could now offer grad students a hassle-free research environment with a real chance of academic employment later on. This may have been due to avoidance of the stormy issue of population differences (although academia in general was becoming calmer). A bigger reason, however, was the strong focus of its main proponents, John Tooby and Leda Cosmides. They talked at conferences, gave interviews to the media, networked, and published, published, published. As they slowly climbed the academic career ladder, they would turn around to help likeminded people on lower rungs.

Bit by bit, they did what the big names could not do. They created a new teaching and research environment. And this environment would be called ‘evolutionary psychology.’


1. Whenever Cavalli-Sforza lists his publications he never goes farther back than 1947. His wartime publications are all the more unknown because they were published under the name of Cavalli. He later changed his name to Cavalli-Sforza, having been ostensibly adopted by the second husband of his maternal grandmother. His autobiography dates the name change to 1950 (a year after his father died) when he was 28, married, and already a father. Such circumstances hardly justified adoption under Italian law or custom. Even more inexplicably, he was still publishing under his old name as late as 1953. Google Scholar lists three publications by L.L. Cavalli in 1950, one in 1951, five in 1952, and two in 1953.


Berry, J.W., and L.L. Cavalli-Sforza. (1986). Cultural and genetic influences on Inuit art. Report to Social Sciences and Humanities Research Council of Canada, Ottawa.

Boyd, R. and P.J. Richerson. (1985). Culture and the Evolutionary Process, Chicago: Chicago University Press.

Cavalli-Sforza, L.L. and F. Cavalli-Sforza (2008). La génétique des populations : histoire d'une découverte, Paris: Odile Jacob. (French translation of Perché la scienza : L’aventura di un ricercatore).

Evolution, Psychology and Psychiatry. (1988). Meeting Announcement, Ann Arbor, Michigan, October 28-30, 1988.

HBES. (1990). Summary of the first Human Behavior and Evolution Conference: Evanston, Illinois, August, 1989, Human Behavior and Evolution Society Newsletter, 1(1), 2-4, July 1990.

Lévi-Strauss, C. (1985). Claude Lévi-Strauss à l’université Laval, Québec (septembre 1979), prepared by Yvan Simonis, Documents de recherche no. 4, Laboratoire de recherches anthropologiques, Département d’anthropologie, Faculté des Sciences sociales, Université Laval.

Stone, L. and P.F. Lurquin. (2005). A Genetic and Cultural Odyssey. The Life and Work of L. Luca Cavalli-Sforza. New York: Columbia University Press.

Symons, D. (1989). A critique of Darwinian anthropology, Ethology and Sociobiology, 10, 131-144.

Tooby, J. and L. Cosmides. (1989). Evolutionary psychology and the generation of culture, Part I. Theoretical considerations, Ethology and Sociobiology, 10, 29-49.

van den Berghe, P. L. & P. Frost. (1986). Skin color preference, sexual dimorphism, and sexual selection: A case of gene-culture co-evolution? Ethnic and Racial Studies, 9, 87-113.

Saturday, August 20, 2011

Can evolutionary psychology evolve?

The Environment of Evolutionary Adaptedness, over a million years ago in the Pleistocene. A founding myth of evolutionary psychology.

In the future, how will we look at evolution and human behavior? Perhaps we’ll still be looking through the lens of evolutionary psychology, albeit a more “evolved” one than the current variety. Or perhaps there will be a new paradigm.

One thing is sure. Evolutionary psychology, as now defined, is untenable. It suffers from several flawed assumptions:

- Human nature is uniform, except for gender differences. It came into existence over a million years ago during the Pleistocene, in the Environment of Evolutionary Adaptedness (EEA). This was long before modern humans began to spread out of Africa some 40,000 years ago and eventually form the populations we know today.

- Given the complexity of human behavior, its genetic basis could not have changed to any appreciable extent since the Pleistocene.

- All present-day humans are therefore essentially the same. All differences in behavior, personality, and temperament must result from a single human nature responding to different environmental inputs.

These assumptions are false. Human genetic evolution has actually accelerated over the past 40,000 years, and even more so over the past 10,000. The latter period, in particular, was not one of people adapting to new physical environments defined by climate, landscape, and vegetation. People were adapting to new cultural environments defined by social structure, normative behavior, and technology.

Yes, human behavior is complex, and any genetic influences must be correspondingly complex. But these influences can be radically changed by a few point mutations. There’s no need to start from scratch, as John Tooby and Leda Cosmides imply. There may simply be changes to developmental timing, such as an infant’s mental plasticity being extended into older life stages. Or there may be changes to the degree of masculinization or feminization. The possibilities are endless. Again, there is no need to posit a huge number of genetic changes.

Yes, we are adapted to past environments—and not to the present one. And there is often a mismatch between something that made sense in the past and our present reality. But why assume a time gap of over one million years? Is it because the Pleistocene makes an ideal setting for just-so stories?

I suspect there is another, more cynical reason. By placing the evolutionary origins of human nature in the distant past, one avoids the messy reality of differences among current human populations—differences in outlook, personality, time orientation, and behavioral predisposition. The Pleistocene EEA may be a just-so story about the past, but it has also had a real impact on the present. It was part of the deal that made evolutionary psychology possible, in the wake of the firestorm that consumed sociobiology.

Will evolutionary psychology evolve?

A paradigm can evolve. Medicine was a pseudo-science that killed more patients than it cured as late as the 1920s. In the space of a few decades, the situation completely reversed. There have been similar turnarounds in other fields. Alchemy became chemistry, and astrology became astronomy.

Indeed, there have been calls for a rethinking of evolutionary psychology, even from Tooby and Cosmides. “Although the hominid line is thought to have originated on edges of the African savannahs, the EEA is not a particular place or time” (Tooby and Cosmides, 2005, p. 22). It is a composite of whichever selection pressures brought each adaptation into existence. There are thus potentially as many EEAs as there are adaptations, and some may be later than others.

This year, several evolutionary psychologists authored what may be called a manifesto for change:

We argue that the key tenets of the established EP paradigm require modification in the light of recent findings from a number of disciplines, including human genetics, evolutionary biology, cognitive neuroscience, developmental psychology, and paleoecology. For instance, many human genes have been subject to recent selective sweeps; humans play an active, constructive role in co-directing their own development and evolution; and experimental evidence often favours a general process, rather than a modular account, of cognition. (Bolhuis et al., 2011)

The text parallels my recent paper in Futures, sometimes strikingly so. It starts off by observing that in the early years of evolutionary psychology “our knowledge of the human genome was limited and gradualism dominated evolutionary thinking.” Today, we know differently:

Events in the Holocene (the last 10,000 years), particularly the adoption of agriculture, domestication of animals, and the increases in human densities that these practices afforded, were a major source of selection on our species, and possibly accelerated human evolution. Evidence from the human genome strongly suggests that recent human evolution has been affected by responses to features of the environment that were constructed by humans, from culturally facilitated changes in diet, to aspects of modern living that inadvertently promoted the spread of
(Bolhuis et al., 2011)

This recent evolution has especially shaped the human brain: “Genes expressed in the human brain are well-represented in this recent selection.”

Even when assessed on its own terms, the Pleistocene EEA looks more and more like a myth, and should be treated as such:

[…] the abstract concept of stable selection pressures in the EEA is challenged by recent evidence from paleoecology and paleoanthropology. The Pleistocene was apparently far from stable, not only being variable, but progressively changing in the pattern of variation. The world experienced by members of the genus Homo in the early Pleistocene was very different from that experienced in the late Pleistocene, and even early anatomical modern Homo sapiens that lived around 150,000 years ago led very different lives from Upper Paleolithic people (40,000 years ago) (Bolhuis et al., 2011)

Is a paradigm shift in the offing? Probably. But what form will it take? Perhaps the second question is unimportant. Whether evolutionary psychology changes or disappears, we’ll be looking at evolution and human behavior in a very different light.

To be cont’d


Bolhuis, J.J., G.R. Brown, R.C. Richardson, and K.N. Laland. (2011). Darwin in mind: New opportunities for evolutionary psychology, PLoS Biol 9(7): e1001109. doi:10.1371/journal.pbio.1001109

Frost, P. (2011). Human nature or human natures? Futures, 43, 740–748.

Tooby, J. and L. Cosmides. (2005). Conceptual foundations of evolutionary psychology, in: D. M. Buss (Ed.) The Handbook of Evolutionary Psychology, Hoboken, NJ: Wiley, pp. 5-67.

Saturday, August 13, 2011

Evolution and human behavior: Towards a new paradigm?

Illustration from Darwin’s book The Expression of the Emotions in Man and Animals (1872)

Evolution has shaped not only our anatomy but also our behavior. This was recognized by Charles Darwin himself in his work The Expression of the Emotions in Man and Animals (1872). Not until the early 20th century, however, would evolution and human behavior emerge as a real field of study. It has gone by three successive names so far:


This largely German school began in 1937 with the founding of the journal Zeitschrift für Tierpsychologie, mainly through the efforts of Konrad Lorenz. The initial aim was to understand the evolutionary origins of human behavior by identifying human behavioral patterns, by examining how they develop during the life of an individual, and by comparing them to homologous behaviors in other primates and mammals.

From the 1960s onward, ethology increasingly shunned the study of human behavior, ostensibly because only nonhuman species could be observed under controlled conditions. Once we had fully understood how and why they behave, it would then be possible to move on to humans. This self-imposed limitation became self-reinforcing: only zoologists went into this field of study, and the occasional musings about human behavior tended to be amateurish.

When did ethology cease to mobilize research into human behavior? The cut-off date probably lies shortly after Lorenz’s death in 1989 and the publication of Human Ethology by his student Eibl-Eibesfeldt the same year.


Launched in the late 1970s, this North American school drew heavily on the latest developments in evolutionary thinking, which in turn drew on economics and game theory. From the outset, it ran into a firestorm of opposition that doomed any real chances for growth or even survival.

Even under better circumstances, it is doubtful whether this field of study could have survived without serious rethinking. Its main shortcomings were:

- “presentism,” a tendency to see contemporary human behavior as an adaptation to present environments, however recent or novel they might be.

- a resulting tendency to see modern behavior as being necessarily adaptive, despite evidence to the contrary. Falling birth rates, for instance, were attributed to parents switching to a K-type reproductive strategy.

- a disinterest in the actual pathways by which genes influence behavior. Such influences were said to be unknowable. Instead, sociobiologists preferred to invoke a mysterious “fitness-maximizing mechanism.”

- naïve and often impressionistic use of anthropological data

By the 1990s, few sociobiologists wished to identify themselves as such, if only because the term itself had become an obstacle to public acceptance. In 1997, this field did away with itself. Its leading journal, Ethology and Sociobiology, was renamed Evolution and Human Behavior.

Evolutionary psychology

This school is likewise North American and largely based in California, its leading pioneers being Leda Cosmides, John Tooby, Don Symons, and David Buss. It took over from sociobiology in the mid-1990s and still dominates thinking on the evolutionary origins of human behavior. Its writers come overwhelmingly from psychology and to a lesser extent from anthropology, biology, and even the humanities. Psychologists are over-represented largely because their discipline better weathered the anti-sociobiology firestorm of the 1980s.

In reaction to sociobiology, evolutionary psychology sought to understand the actual ways in which genes influence behavior, which was now seen as an adaptation to past environments. This school developed around four basic principles:

1. The environment of evolutionary adaptedness (EEA). Past environments have shaped human nature. How far back in the past is not clearly stated, although the EEA has typically been equated with the savanna of Pleistocene Africa.

2. Gradualism. The human mind is a product of co-adapted gene complexes that cannot respond quickly to selection. Like the EEA, gradualism denies that human nature could have evolved differently in the different cultural and physical environments that modern humans entered as they spread out of Africa over the past 40,000 years

3. Modularity of the human mind. Because specific adaptive problems require specific adaptive solution, the human mind is mainly composed of domain-specific, modular programs.

4. Universal human nature. There is only one human nature. Apparent differences in human nature are simply different outcomes of species-wide programs (as a result of different environmental inputs).

The above principles are more than a reaction against sociobiology. They’re an overreaction. Some genetic determinants of human behavior are clearly post-Pleistocene in origin. How are we supposed to explain them? And what about more domain-general aspects of the human mind, like general intelligence?

But, then, this “overreaction” isn’t just a matter of rational argument. There is also fear. Leda Cosmides and John Tooby remember the firestorm that ravaged sociobiology in the 1980s. They then wandered a long time in the wilderness before finally landing a permanent academic position. Time and again, they had to convince potential employers or funding agencies that they had no secret interest in psychological differences among human populations.

And so was paved the road to evolutionary psychology, a road paved with the best of intentions: legitimate criticisms of sociobiology, with an understandable desire to lead a normal academic life.

This desire is described by Cosmides and Tooby in one of their articles:

The Standard Model therefore frees those in the biological sciences to pursue their research in peace, without having to fear that they might accidentally stumble into or run afoul of highly charged social or political issues. It offers them safe conduct across the politicized minefield of modern academic life. This division of labor is, therefore, popular: Natural scientists deal with the nonhuman world and the “physical” side of human life, while social scientists are the custodians of human minds, human behavior, and, indeed, the entire human mental, moral, political, social, and cultural world. (Barkow, Cosmides, & Tooby, 1992)

Ironically, in seeking to challenge this modus vivendi, Cosmides and Tooby have ended up creating a new one. We can now study human behavior from an evolutionary perspective, but only if we exclude (1) psychological differences between human populations and (2) domain-general aspects of human behavior, e.g., general intelligence. That’s the implicit deal behind evolutionary psychology—an acquiescence to self-censorship.

How has the deal worked out? Quite well, apparently. Evolutionary psychology has attained a degree of respectability that seemed impossible back in the mid-1990s. In 1995, only 261 newly published academic books or articles mentioned the term “evolutionary psychology.” Last year, the figure was 3,570 (see Google Scholar). This field now has its own handsomely illustrated textbooks, grad school programs, and research centers. Perhaps more importantly, it now incurs few costs to a career in academia.

But maybe this would have happened anyway. By the mid-1990s, the anti-sociobiology firestorm was burning itself out. The far left had entered a steep decline, and its graying leadership was pushing sixty. With rising tuition, a weakening economy, and aging demographics, the social sciences were attracting fewer and better students. Meanwhile, the advent of the Internet was “deregulating” the marketplace of ideas.

Has self-censorship calmed debate over evolution and human behavior? I’m not so sure. In any case, it has certainly done much to distort the way the debate is framed. And this is the sad part. Evolutionary psychologists often produce interesting findings, only to fall down when they try to interpret them.

For instance, it is known that children develop differently if the biological father is absent and a strange male is present (e.g., a stepfather). In both sexes, sexual activity will begin earlier with less stable pair bonds. Sons will show hypermasculine behavior, such as aggressive acting out, boasting, and risk-taking. Daughters will reach puberty earlier and judge potential mates by current appearance and status in the male hierarchy rather than by steadfastness and ability to support a family. It has thus been hypothesized that an early sensitive period allows certain environmental cues, like father presence, to define reproductive strategy later in life (Ellis et al., 2003).

The above reasoning is consistent with the four principles of evolutionary psychology. It’s also false. A recent twin study has found that early menarche is predicted as strongly by a step-uncle’s presence as by a stepfather’s. “It does not seem necessary for a child to experience the direct environmental influence of a stepfather to exhibit an accelerated age of menarche—as long as she is genetically related to someone who does have a stepfather” (Mendle et al., 2006).

In other words, a woman may be more prone than others to early menarche, a high degree of female reproductive autonomy, and low expectations of paternal investment. It’s not as if she acquires this reproductive strategy from her childhood environment. Instead, she inherits it genetically from her mother and absent father.

So where do we go from here? We can continue down the road paved by Cosmides and Tooby. But it will often take us to a dead end. We’ll then have to waive some or all of the above four principles, assuming of course we wish to understand human behavior.

To be cont’d


Barkow, J.H., Cosmides, L., & Tooby, J. (eds.) (1992). The Adapted Mind. Evolutionary Psychology and the Generation of Culture, New York, Oxford: Oxford University Press.

Ellis, B.J., J.E. Bates, K.A. Dodge, D.M. Fergusson, L.J. Horwood, G.S. Pettit, L. Woodward. (2003). Does father absence place daughters at special risk for early sexual activity and teenage pregnancy? Child Development, 74, 801-821.

Mendle, J., E. Turkheimer, B.M. D’Onofrio, S.K. Lynch, R.E. Emery, W.S. Slutske, and N.G. Martin. (2006). Family structure and age at menarche: a children-of-twins approach, Developmental Psychology, 42, 533-542.

Saturday, August 6, 2011

Brain size, latitude, and ambient light

An Inuk wearing snow goggles. Is ambient light at its lowest in Inuit territory?

The logjam seems to have broken. On the heels of Lewis et al. (2011), we now have another paper on variation in brain size among human populations, this time by Pearce and Dunbar (2011).

Brains vary in size by latitude, being bigger at higher latitudes and smaller at lower ones. This variation seems to reflect an adaptation to climate. But just how, exactly, does climate relate to brain size? How direct or indirect is the relationship?

Pearce and Dunbar (2011) argue that bigger brains are an adaptation to lower levels of ambient light. Specifically, dimmer light requires larger eyes, which in turn require larger visual cortices in the brain. Using 73 adult crania from populations located at different latitudes, the two authors found that both eyeball size and brain size correlate positively with latitude. The correlation was stronger with eyeball size, an indication that this factor was driving the increase in brain size.

How credible is this explanation? First of all, visual cortex size was not directly measured. The authors inferred that this brain area was responsible for the increase in total cranial capacity. Obviously, they couldn’t have done otherwise. They were measuring skulls, not intact brains.

But there’s another problem—one in the realm of logic. A lot of things correlate with latitude: pigmentation, mating systems, rules of descent, degree of paternal investment, and so on. If one of them correlates more strongly with latitude than the others, does it therefore cause the others? Not at all. It may be closer than the others to this shared cause, but it doesn’t necessarily lie on the same causal chain as the others.

In other words, the level of ambient light does not produce a single cascade of consequences, with eyeball size being the first consequence. There are probably many different cascades.

To date, the best map of human variation in brain size is the one by Beals et al. (1984) (see previous post). If dimness of light is the main determinant, brain size should be highest in northwestern Europe, northern British Columbia, the Alaskan panhandle, and western Greenland. These regions combine high latitudes with generally overcast skies. Yet they are not the regions where humans have the biggest brains. Instead, brain size is at its highest among humans from the northern fringe of Arctic Asia and from northeastern Arctic Canada. These regions are, if anything, less overcast than average. They often have high levels of ambient light because of reflection from snow and ice.

The jury is still out on this question. I suspect, however, that the following three factors probably explain variation in brain size with latitude.

1. Among hunter-gatherers, hunting distance increases with latitude because there are fewer game animals per square kilometer (Hoffecker, 2002, pp. 8-9). Hunters must therefore store larger amounts of spatiotemporal information (landmarks, previous hunting itineraries, mental simulations of possible movements by game animals over space and time). This factor might explain why brains have grown smaller since the advent of agriculture.

2. The seasonal cycle matters more at higher latitudes. As a result, northern hunter-gatherers, and northern agriculturalists even more so, must plan ahead for the next season (or even for the season after the next one).

3. Women gather less food at higher latitudes and almost none in the Arctic. They are thus free to specialize in other tasks, such as garment making, food processing, and shelter building. This “family workshop” creates opportunities for greater technological complexity, which in turn increases selection for greater cognitive performance.

I suspect bigger brains provide not so much greater intelligence as greater ability to store information. As such, they nonetheless pre-adapted northern hunter-gatherers for later advances in cultural evolution.


Beals, K.L., C.L. Smith, and S.M. Dodd (1984). Brain size, cranial morphology, climate, and time machines, Current Anthropology, 25, 301–330.

Hoffecker, J.F. (2002). Desolate Landscapes. Ice-Age Settlement in Eastern Europe. New Brunswick: Rutgers University Press.

Lewis, J.E., D. DeGusta, M.R. Meyer, J.M. Monge, A.E. Mann, R.L. Holloway. (2011). The Mismeasure of Science: Stephen Jay Gould versus Samuel George Morton on Skulls and Bias, PLoS Biology, 9(6) e1001071

Pearce, E. and R. Dunbar. (2011). Latitudinal variation in light levels drives human visual system size, Biology Letters, doi: 10.1098/rsbl.2011.0570