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March 25, 2005

Male brain ~ more sons vs. female brain ~ more daughters?

Doing some reading on the Trivers-Willard hypothesis I found this: Engineers have more sons, nurses have more daughters: an evolutionary psychological extension of Baron–Cohen's extreme male brain theory of autism:

In his extreme male brain theory of autism, Baron-Cohen postulates that having a typically male brain was adaptive for ancestral men and having a typically female brain was adaptive for ancestral women. He also suggests that brain types are substantially heritable. These postulates, combined with the insight from the Trivers–Willard hypothesis regarding parental ability to vary offspring sex ratio, lead to the prediction that people who have strong male brains should have more sons than daughters, and people who have strong female brains should have more daughters than sons. The analysis of the 1994 US General Social Survey data provides support for this prediction. Our results suggest potentially fruitful extensions of both Baron-Cohen's theory and the Trivers–Willard hypothesis.

I don't have access to the whole article, so be forewarned (I don't know if the authors are simply observing a correlation between "male brain" professions with high SES, which would tend correlate with a male-skewed sex ratio according to the Trivers-Willard hypothesis).

Support for the Trivers-Willard hypothesis seems stronger in animals, especially in those that are highly polygynous (greater male reproductive variance, so a bigger payoff for mothers who can "invest" well in sons so they exhibit more attractive phenotypes).

Update Well...since someone asked, I did remember this paper: Offspring sex ratio in women with android body fat distribution:

...After statistically controlling for subject's age, socioeconomic status, and total number of offspring, we found that women with a higher WHR tended to have more sons than daughters. In addition, women who reported greater ease of having multiple orgasms also tended to have more sons than daughters. The results thus support both a hormonal and a behavioral influence on offspring sex ratio.

Baron-Cohen says that the "male brain" is correlated with lower-than-median levels of testosterone in males and higher-than-median levels of testosterone in females. High Waist-Hip-Ratio (WHR) tends to be found in women with higher relative rates of testosterone. The research above (which has been disputed!) suggests that there is a male bias in sex ratio at birth for women who have higher WHR, ergo, higher testosterone levels. Of course this leaves the males out of the equation, but the Trivers-Willard hypothesis is generally concerned with female parental strategies in any case. And don't discount sex-selective abortions....

Addendum: Testosterone probably plays a role in sex drive as well, so don't discount the orgasm result being connected to the WHR.

Related: The essential difference.

Posted by razib at 01:34 PM