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| Diorama of Homo erectus at the National Museum of Mongolian History; source: Wikimedia Commons. |
An unfair proclamation I commonly hear is that evolutionary psychology should be dismissed because its theories have never been tested according to their predictive power. The criticism goes,
Evolutionary psychologists only speak of what happened in the past. But if it were a real science, then the principles it describes should apply consistently. And if the principle applies consistently, evolutionary psychologists should be able to apply that principle to make predictions in experiments. That doesn’t happen. Therefore, evolutionary psychology consists entirely of just-so stories wherein evolutionary psychologists observe some current social arrangement and then work backward, coming up with some story to explain how that arrangement came about.
I won’t argue that evolutionary psychology is perfect. Often evolutionary psychologists do come up with rationalizations for social and political collectivism. As I have pointed out before, many enthusiasts of evolutionary psychology, such as Jonathan Haidt and Michael Shermer, try to cite evolutionary psychology to proclaim that biology itself disproves Ayn Rand’s ethical theory. And even after frequently acknowledging that voluntary trades are win-win positive sum games, many evolutionary psychologists still subscribe to a fallacious neo-Malthusian economic paradigm that presumes that human beings under free enterprise will fritter away all nonrenewable natural resources and destroy themselves.
However, it is actually untrue that evolutionary psychology has not been applied to making accurate predictions. Zoologist Marlene Zuk provides several examples in Riddled With Life: Friendly Worms, Ladybug Sex, and the Parasites That Makes Us Who We Are, (Orlando, FL: Harcourt, 2007).
Predictions and Tests of Evolutionary Psychologists’ Theories on Polygyny
Here is one example of a longstanding theory in evolutionary psychology. There are species that are called polygynous. The more polygynous a species is, the more it is a “harem species.” This means that many males compete against one another in displays of machismo. The male that wins the competition gets to have sex with many females, and the males that lose the competition will die without having mated at all. Some species are more severely polygynous than others, and evolutionary psychologists tend to describe our species as “mildly polygynous” (this is one reason why so many feminists disparage evolutionary psychology as one big rationalization for patriarchal gender norms).
The more aggressively polygynous a species is, the more different the males will look from the females -- the extent to which males and females of the same species look different is called sexual dimorphism. Generally if a species is aggressively polygynous, either the male is much more colorful than the female (males compete for female attention according to how they look; this is in birds) or the male is much larger than the female (as the males outright fight one another).
Evolutionary psychology continues that the more polygynous a species is, the more testosterone the males produce -- the testosterone contributes to the competitive aggression. Simultaneously, testosterone production weakens the immune system. This means that if the evolutionary psychologists’ theory about polygynous species is true, then -- everything else being equal -- the more polygynous a species is, the more vulnerable old-age males of the species will be to parasites than females of the same age. Note that this theory was developed before scientists put the theory to the test in experiments. And Marlene Zuk describes how this theory was indeed put to the test. Was the prediction accurate?
Sarah Moore and Ken Wilson from England found an ingenious way around the problem using a simple surrogate for the degree of male reproductive competition: the size differential between the sexes. Species in which the males are relatively larger are presumed to have experienced more male sexual competition in their evolutionary history. They used information available in the scientific literature on body size and parasites from 106 different mammals ranging from deer to elephants to mice, and they also used sophisticated methods for taking into account the ancestral relationships between species. The parasites included single-celled organisms, worms, mites, fleas, and ticks. Viral and bacterial infections, though potentially extremely important in the lives of the animals, are simply too difficult to document in wild populations, so Moore and Wilson left them out of the analysis.
Moore and Wilson first found the same pattern that had been established in smaller studies: Males had persistently higher levels of parasitism than females. As they predicted, the greater the difference in size between males and females, the greater the disparity between male and female parasite levels. The scientists also gathered data on longevity in the sexes, and found that when males died at a younger age, they also had a disproportionately higher level of disease. Furthermore, where they could at least classify species into those in which males had the potential to mate with more than one female and those that were monogamous, Moore and Wilson showed that male-biased parasitism was more likely in the former. This is consistent with the idea that intense male competition leads to males being the sicker sex. [Marlene Zuk cites Sarah L. Moore and Ken Wilson, "Parasites as a Viability Cost of Sexual Selection in Natural Populations of Mammals," Science vol. 297, year 2002: pages 2015-2018.]
Those were from pages 136–37 of Marlene Zuk’s book.
You may recall my mentioning that William Donald “W.D.” Hamilton is the scientist who coined kin selection. With W. D. Hamilton, Marlene Zuk conducted her own experiment to test this theory. The brightness of a male peacock’s feathers indicates its resistance to disease. If a peacock is very bright blue, that does not indicate the absence of parasites in its system. Rather, the peacock’s bright blueness indicates that there are many parasites in the peacock’s system but that its immune system is so strong, the peacock is able to survive in spite of the parasites anyway. According to this understanding, then, evolutionary psychology predicts that the brighter blue a male peacock is, the more parasites should be found in its family history. Marlene Zuk and W. D. Hamilton ran a test on this and found that the evolutionary psychologists’ theory proved correct. (William D. Hamilton and Marlene Zuk, “Heritable True Fitness and Bright Birds: A role for Parasites?”, Science vol. 213, year 1982: pages 384-387.)
Does the Evolutionary Psychologists’ Prediction About Polygyny Apply to Human Societies As Well? Another Test
Moreover, evolutionary psychologists such as Bobbi S. Low have found that this theory has accurately predicted findings in human societies. According to evolutionary psychology theory, it is the case that -- everything else being equal -- the more severely polygynous a particular human society is, the more parasites will be found in the systems of the men. Is that the case?
That part, of course, could still be important, and Bobbi Low, a biologist at the University of Michigan, looked at human societies around the world to see whether parasites were more prevalent in cultures where polygyny, the practice of a single man having many wives or concubines, was more common. She reasoned that this was similar to our prediction about heavier parasite burdens leading to higher degrees of sexual selection and hence showier plumage in the birds. Indeed, societies where polygyny is common are more plagued by diseases such as malaria, sleeping sickness, and various worms.
That is from pages 160–61, citing Bobbi S. Low, “Marriage Systems and Pathogen Stress in Human Societies,” American Zoologist vol. 30, no. 2, year 1990: pages 325-339.
Evolutionary Psychologists’ Theory About Collectivism, Xenophobia, and Disease
Here is another evolutionary psychology theory -- one developed by W. J. Freeland in 1976: it is that particularly insular, collectivist, xenophobic societies are that way on account of communicable disease. The theory is that they started out as rather open to contact and trade with foreigners but, as a consequence, they contracted communicable diseases. The members of the society who survived the original epidemic were able to maintain their society by becoming less welcoming toward contact with foreigners and also more keen on enforcing social conformity.
If that theory were correct, then it follows that if you check the history of communicable diseases among various societies, the most insular and xenophobic of societies should also be the ones with the most severe histories of communicable diseases being passed from person to person. In 2009, a Canadian team led by Randy Thornhill conducted such an investigation. The team’s findings corroborated Freeland’s theory. See Randy Thornhill, et all., “Parasites, Democratization, and Liberalization of Values Across Contemporary Cultures,” Biological Reviews vol. 8 (no. 1, February 2009): pages 113–133.
Conclusion
Yes, evolutionary psychology theories have been used to make accurate predictions in experiments concerning human behavior and human societies.
The problems with evolutionary psychology normally come from the evolutionary psychologists’ misinterpretations of their own data. As I wrote of previously, evolutionary psychologists frequently observe the trend that hunter-gatherer societies foster a collectivist interpretation of ethics and morality, and thereby conclude that a modern, liberalized, industrial society also ought to be collectivist, making an ethics of peaceable self-interest unworkable. However, it is terribly inaccurate to proclaim that evolutionary psychology theory has never made accurate predictions that scientists have tested. No, evolutionary psychologists’ theories on how specific behaviors arose do not amount to mere “just-so stories.”
