Evolution’s influence on personality?

Evolutionary psychology is an approach to understanding human personality traits that I’m a little skeptical of. Evolutionary psychologists use Darwin’s discovery that an organism will develop mechanisms that facilitate its reproduction over time as justification for the view that the brain may similarly evolve in favor of traits that increase reproductive success.

One of the many topics that evo psych studies investigate is whether humans have evolved the capability to indirectly affect the sex of their offspring in order to produce offspring that will have maximal reproductive success in the environment in which they are born, known as the Trivers-Willard Hypothesis (TWH) (other animals do seem to have such a capacity). Specifically, the TWH states that when male fitness variance exceeds female fitness variance for a certain trait (i.e., when males who lack the trait have significantly less reproductive success than females who lack the trait), parents in better conditions are expected to produce more male offspring than females. Therefore, weak populations should alter their offspring ratio in favor of females, since female offspring will experience less of a reproductive setback based on their conditions than male offspring would.

Not surprisingly, findings are mixed. One study, cleverly titled “You are what your mother eats,” found, by examining food diary data from 740 pregnant British women, that mothers’ overall energy intake was positively correlated with producing male children. When the data were divided into three categories that represented the lowest, middle, and highest number of calories consumed, 56% of the women in the highest category had boys, while only 45% of the women in the lowest category did, a significant difference.

Oddly enough, the researchers also found that the amount of cereal a woman ate was also positively correlated with her chances of bearing a son. This is attributed to the fact that cereal is the most common breakfast food in the UK, so many of the women not eating cereal were not eating breakfast at all, and in turn were consuming fewer total calories than the cereal-eaters. Additionally, skipping breakfast decreases the amount of glucose in the body, so the researchers conjectured that the lower levels of glucose were interpreted by the body as overall lower resource conditions. In short, they argued that these findings demonstrate that a mother’s nutritional condition seems to be a significant predictor of her offspring’s sex.

Another study, titled “Beautiful parents have more daughters,” argues that physical attractiveness, “defined by the geometric concept of bilateral symmetry, the mathematical concept of averageness, and the biological concept of secondary sexual characteristics,” affects females’ reproductive success more than males’, since females tend to prioritize traits like stability and status for a male mate, while males prioritize attractiveness in their mates. The researchers rated parents’ attractiveness on a 5-point scale, and found that participants in the top category had significantly lower chances of bearing sons than parents in the bottom 4 categories, those who were less attractive, did. Again, they concluded that since attractiveness is a more important quality in female offspring, those parents who were highly attractive altered their offspring sex ratio in favor of females, the gender who would benefit more from the attractive phenotype.

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However, many studies fail to find evidence of the Trivers-Willard Hypothesis in offspring sex ratios. Since females use status as a criterion for male mate selection, one study predicted that parents of high socioeconomic status should be more likely to have male children, but found that this was not the case.

Because biological traits that increase individuals’ chances for reproduction are selected for, it seems logical that psychological traits may evolve in a similar way. This, in addition to the fact that humans have a propensity to draw causal relationships in almost every area of life, makes evolutionary psych very seductive. However, it is likely not a complete explanation for personality traits. One major shortcoming of this argument is that some traits that drastically reduce individuals’ chances for reproduction, such as homosexuality, remain common phenotypes. Evo psych also forces proponents to take a reductionist view of the mind, tending to reduce it directly to the sum of its parts, the physical brain. To me, it feels that context is often missing from evolutionary explanations of psychological traits, at least as far as the TWH is concerned. Could it really be possible that individual environmental traits can significantly influence a woman’s chances of bearing a son or a daughter?


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