As used in this article, pop evolutionary psychology, or Pop EP, refers to a branch of theoretical psychology that employs evolutionary principles to support claims about human nature for popular consumption.Fallacy 1: Analysis of Pleistocene Adaptive Problems Yields Clues to the Mind's Design
Tooby and cosmides have argued that because we can be quite certain that our Pleistocene ancestors had to, among other things, “select mates of high reproductive value” and “induce potential mates to choose them, ” we can also be sure that psychological adaptations evolved for solving these problems. But efforts to identify the adaptive problems that drove human psychological evolution confront a dilemma.
On the one horn, while it is true that our ancestors had to “induce potential mates to choose them, ” for example, such a description is too abstract to provide any clear indication of the nature of human psychological adaptations. All species face the problem of attracting mates. Male bowerbirds build ornately decorated bowers, male hangingflies offer captured prey, and male sedge warblers sing a wide repertoire of songs. Figuring out which strategies ancestral humans had to use requires a much more precise description of the adaptive problem for early humans.
More precise descriptions of the adaptive problems our ancestors faced, however, get impaled by the other horn of the dilemma: these descriptions are purely speculative because we have little evidence of the conditions under which early human evolution occurred. The paleontological record provides a few clues about some aspects of early human life, but it is largely silent regarding the social interactions that would have been of principal importance in human psychological evolution. Nor do extant hunter-gatherer populations provide many hints about the social lives of our ancestors. Indeed, the lifestyles of these groups vary considerably, even among those who live in the regions of Africa that had been populated by early humans.
Moreover, as biologist Richard Lewontin of Harvard has argued, the adaptive problems faced by a species are not independent of its characteristics and lifestyle. Tree bark contributes to the adaptive problems faced by woodpeckers, but stones lying at the foot of a tree do not. In contrast, for thrushes, which use stones to break snail shells, the stones are part of the adaptive problems they face, whereas tree bark is not. Similarly, our ancestors' motivational and cognitive processes would have been selectively responsive to certain features of the physical and social environments, and this selective responsiveness would have determined which environmental factors affected human evolution. So to identify the adaptive problems that shaped the human mind, we need to know something about ancestral human psychology. But we don't.
Finally, even if we could precisely identify the adaptive problems faced by our ancestors throughout human evolutionary history, we still couldn't infer much about the nature of human psychological adaptations. Selection builds solutions to adaptive problems by retaining modifications to preexisting traits. Subsequent adaptation is always a function of how preexisting traits were modifiable. To know how a solution to an adaptive problem evolved, then, it is necessary to know something about the preexisting trait that was recruited and modified to solve the problem. Without knowledge of our ancestors' psychological traits—which we don't have—we can't know how selection tinkered with them to create the minds we now possess.Fallacy 2: We Know, or Can Discover, Why Distinctively Human Traits Evolved
Biologists are often able to reconstruct the selection pressures that drove a species' evolution by using the comparative method to study a clade, or group of species descended from a common ancestor. Because all the species in the group are descended from a common form, differences among them may be the result of variations in the environmental demands they faced. When a trait is shared by two or more species in a clade, but not by the others, it is sometimes possible to identify environmental demands common to those species but absent among the species without the trait. Correlating trait differences with specific environmental variations, in this way, can indicate the environmental demands to which a trait is adapted.
But the comparative method offers little help for Pop EP's aspiration to reveal the adaptive history of the psychological traits—including language and forms of higher cognition—that putatively constitute human nature. Pinker, for example, has argued eloquently that language is an adaptation for verbal communication of infinite combinatorial complexity. He is probably right that language is an adaptation. But discovering why it evolved—what it is an adaptation for—requires identifying the adaptive functions that language served among early language users. To employ the comparative method to answer such questions, we need to compare some human psychological trait with its homologous form in species with whom we share a common ancestor. Here looms the problem. Among extant species, our closest relatives are the chimpanzee and the bonobo, with whom we share a common ancestor that lived approximately six million years ago. But even these, our closest relatives, don't possess forms of the complex psychological traits, such as language, whose evolution Pop EP aspires to explain. So we can't identify the environmental demands we share with our closest relatives to see what our common psychological traits are adapted to. Rather we need to identify the environmental demands that drove our evolutionary separation from our closest living relatives during the past six million years.