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:

Ethology

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.

Sociobiology

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

References

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.