Why do we do the things we do?

Could evolution explain our innermost fears and motivations? Some scientists think so, while others are less confident. One thing is certain though—the budding field of evolutionary psychology is highly controversial.

Evolutionary biology is the study of how organisms have diversified and changed by means of natural selection over geological timescales. Psychology is the study of human mental processes and behaviors. Evolutionary psychologists aim to combine these two disciplines to provide a biological framework that explains human motivations and fears based on our evolutionary past.

“I am interested in understanding deeper questions about the origins of the human mind,” said David Buss, professor of psychology at the University of Texas at Austin and an evolutionary psychology pioneer. “The motives, the fears, the drive we have, from our proclivity to collaborate to our adaption to compete.”

In hindsight, Freud’s theory of psychology highlighting the role of sexuality in human behavior and William James’ psychology based on instincts count among the founding principles of the field of evolutionary psychology. The goal is ambitious, and the field is gaining notoriety. The colloquial use of the term “human nature” to explain someone’s behavior attests to this.

But progress in evolutionary psychology has been hampered by controversy. Recently, a number of philosophers, biologists, sociologists, and psychologists summarized their concerns about the field’s methods and its potentially harmful social ramifications in the book Alas Poor Darwin (1). They expressed concern that evolutionary psychologists rely on a priori hypotheses and that their conclusions might be used to justify existing social hierarchies. Some take criticism even further: “[Evolutionary psychology] is about inscribing your own social and political prejudices onto human evolution, reading them back out, and claiming to speak for evolution,” said Jonathan Marks, professor of anthropology at the University of North Carolina, Charlotte.

Traits that survive the winnowing of natural selection—evolution’s key actor—are either neutral, with no consequence on an organism’s fitness, or positively selected due to the benefits they confer. Evolutionary psychology’s goal of explaining human behavior through the lens of evolution therefore implicitly carries the notion that our behaviors are adaptive because they persist to this day. While Buss doesn’t think that our current behavior is necessarily adapted to our environment, he believes that “All hypotheses about human psychology in the future will be evolutionary ones.” So, could human psychology be boiled down to an evolutionary byproduct?

Contentious Hypotheses

No part of evolutionary psychology is more controversial than mating behavior and interactions between the sexes. Many scientists view our standards of attractiveness and sexual behavior as based more on social norms than biology (2–4), since beauty standards have varied vastly over time: for example, the plump bodies of Titian’s paintings contrast with the thinner standards heralded in today’s magazines.

For researchers highlighting the role of society in behavior, the parallel drawn between animal mating behavior and human mating behavior is a case of analogy mistaken for homology. For instance, they argue that human sexual behavior cannot be driven purely by the goal of reproduction, given that today’s humans widely use various means of preventing pregnancy. Our societal norms and higher-order consciousness have the potential to confound the role biology plays in the evolution of our behaviors.

A lack of evolutionary methodology and the perceived tendency for self-fulfilling prophecies is what rubs Marks the wrong way. For instance, a trait commonly used to show evidence of sexual selection in primate species is the difference in canine tooth size between males and females. “I was reading an article on human mate choice, which failed to acknowledge that human canine teeth indicate the absence of classical primate sexual selection [in humans], since they are non-dimorphic,” highlighted Marks.

But for other scientists, such as Buss, it’s a different story. He’s produced a number of studies about mating psychology that have helped shape the field. “Before we started studying mating, there was no such field. Over the ensuing decades, it really mushroomed,” said Buss. “Now there are thousands of articles on human mating.” His work ranges from dissecting standards of female attractiveness (5) to providing evolutionary explanations for jealousy (6) and friendship (7).

In a recent study, Buss’s team looked at the role of lumbar curvature in perceived female attractiveness (5). The researchers hypothesized that women whose ancestors had shifted their centers of gravity by increasing lumbar curvature would be better adapted for pregnancy loads. In turn, these women should be more attractive to potential mates.

To test this, Buss first asked men of undisclosed sexual preference to rate the attractiveness of images of female models generated by Photoshop, varying in lumbar curvature only. Based on the ability to carry pregnancy loads, Buss hypothesized that the optimal angle of lumbar curvature was 45.5°. He found that men preferred greater lumbar curvature, but often, the preferred angle was much higher than the hypothesized optimum.

Evolution may have played a role in this physical feature of women’s bodies. But the latter results and their departure from the expected optimal lumbar curvature angle fall just short of supporting the conclusion that male preference of attractiveness is due to perceived childbearing capacity.

The Role of Evolution in Competition

Like Buss, James Roney, an associate professor of psychology at the University of California, Santa Barbara, is an evolutionary psychologist interested in mating behavior. His research focuses on courtship mechanisms as well as partner decisions between and among the sexes, with his team recently publishing a study on the effect of menstrual cycle stages on competitiveness between women (8).

“For a couple of years, I have been thinking about female intra-sexual competition,” said Adar Eisenbruch, first author of the study. He asked women to play a modified version of the ultimatum game, a model used in behavioral economics to study cooperation. This game requires two players: the proposer and the responder. The proposer receives a set amount of money and is asked to offer a portion of it to the responder. The experimental observer then lets the responder know how much they were offered and lets them accept or reject it. If the responder accepts the deal, they receive the offered sum, and the proposer keeps the rest. If they refuse the offer, neither player receives any money.

Eisenbruch used a modified set-up called the strategy method, in which the responder does not accept or reject a specific offer but rather states what the lowest offer they would accept is. During his study, he asked the participating women what day of their menstrual cycle they were on.

He found that women demanded more from the proposer at peak ovulation. Eisenbruch interpreted this to mean that heightened pregnancy risk made women less cooperative, demanding more from other women in order to agree to cooperate. “It’s been shown that women treat more attractive men better, and vice versa. It’s courtship. In order to exclude the possibility of mating motives confounding our data, we only looked at female-to-female interactions,” he said.

The Naturalistic Fallacy

“[Evolutionary psychologists generate] results from data without performing adequate controls, which serve in science to constrain possible interpretations of the data. Does anyone not know that human thought is the product of (1) evolutionary heritage, (2) the combination of class, ethnicity, and history, and (3) personal experiences? To try to understand your data as simply the product of (1) is beyond naïve,” warned Marks.

Beyond the methodological concerns lie ethical ones. The belief that what is biological is good and should help shape our society is known as the naturalistic fallacy. Most of the contention with evolutionarily psychology stems from a perceived championing of this viewpoint.

However, some scientists within the field explicitly speak out against this belief. “Evolutionary psychology is a purely descriptive scientific field,” said Eisenbruch. “It’s a huge problem and an embarrassment for the field that there are groups who love to abuse evolutionary psychology. The fact that something evolved in a particular environment does not tell us anything about whether things should or should not be that way.”

“We can never be free of our ideologies, but we can strive to make them more subtle and more benign,” admitted Marks. “But when someone tells you that their work on human nature or origins or behavior is free of ideology, that’s the time to check your wallet because you are about to have your pocket picked.”

Evolutionary psychology’s aim to explain how and why we behave the way we do is very appealing. But this budding field, which is rooted in early twentieth-century psychology and has passionate proponents and opponents, still has some convincing to do. Only time will tell which side ultimately wins the debate.