Sexually Promiscuous Parents Really DO Have More Sons

Science is a cumulative enterprise. Each generation of scientists creates new knowledge, not from scratch all by itself, but by building on the knowledge created and accumulated by previous generations of scientists. That’s why it is important, before starting a new project or writing a new paper, to conduct a thorough literature review, in order to find out what is already known on the topic, so as not to reinvent the wheel. At least that’s what we teach our graduate students. But occasionally scientists make mistakes and miss relevant studies published in their area. I know I did.

As I explain in an earlier post, in my 2009 article with Péter Apari published in the Annals of Human Biology, I derive yet another prediction from the generalized Trivers-Willard hypothesis (gTWH). Unrestricted sociosexual orientation – being sexually promiscuous – disproportionately benefits sons. Sexually promiscuous sons can potentially have a large number of offspring, whereas sexually promiscuous daughters can have no more children than their less promiscuous counterparts with a steady partner. In addition, sexually promiscuous women would have a very difficult time convincing the fathers of their children to make parental investment in their children because the men cannot be assured of their paternity.

So, following the logic of the gTWH, Apari and I predict that sexually promiscuous parents are more likely to have sons than sexually less promiscuous parents. Our analyses of two large representative samples of Americans from the National Longitudinal Study of Adolescent Health (Add Health) and the General Social Surveys (GSS) confirm our prediction.

So far, so good. The only problem is that, long after the article had been published in the Annals of Human Biology, I discover that I had managed to reinvent the wheel! The wheel, in this particular case, had originally been invented by two megasuperstars of evolutionary psychology – Steven W. Gangestad of the University of New Mexico and Jeffry A. Simpson of the University of Minnesota – nearly two decades earlier! Boy, is my face red!

In their 1990 article “Toward an Evolutionary History of Female Sociosexual Variation” published in the Journal of Personality, Gangestad and Simpson predict that sociosexually unrestricted women are more likely to have sons than sociosexually restricted women. The reasoning behind their prediction in 1990 is essentially the same as mine in the gTWH two decades later. Sociosexually unrestricted women are more likely to mate with reproductively highly successful men, and whatever traits such men will pass on to their genetic offspring – including being sociosexually unrestricted themselves – are more likely to benefit sons more than daughters.

In their article, Gangestad and Simpson conduct three studies to test and confirm their prediction. In their first study, they examine the offspring sex ratios of women listed in Who’s Who in American Women. They infer the sociosexuality of women listed in Who’s Who from their occupations. Myers-Briggs publishes the average scores of various personality tests by occupation. Gangestad and Simpson use the traits of “extraversion” and “lack of constraints” as closest analogs of sociosexuality. Their analysis shows that women who are in occupations with the highest extraversion/lack of constraints scores have significantly higher proportions of sons than women in occupations with the lowest extraversion/lack of constraints scores. For example, on the unrestricted sociosexuality end, actresses/performing artists have 63.9% sons and women in marketing and sales have 69.3% sons. On the restricted sociosexuality end, accountants have 50.0% sons and health administrators have 51.7% sons.

In the second study, Gangestad and Simpson use the mothers of undergraduate students as their sample. They assess the mothers’ sociosexuality by using the proxy of the Self-Monitoring Scale (wisely deciding that mothers of undergraduate students will probably not honestly answer questions about their sexual history from their children’s professors!); those who are low in self-monitoring are known to be more sexually unrestricted than those who are high in self-monitoring. Their analysis shows that probable unrestricted mothers had more sons (53.0%) than the probable restricted mothers (44.2%).

Finally, in their third study, they use data collected by Alfred Kinsey and his associates in the late 1940s and 1950s. The data show that the number of premarital sex partners (as a measure of unrestricted sociosexuality) is significantly correlated with the offspring sex ratio (proportion of sons). Unlike the first two studies, Gangestad and Simpson include both men and women in the third study, and their data show that the correlation holds among both men and women (as it does in our study in both Add Health and GSS samples).

I undoubtedly made a major blunder by not discussing Gangestad and Simpson’s 1990 study in my 2009 paper, thereby reinventing the wheel. Nevertheless, it is good to learn (albeit quite belatedly) that our results showing that sexually promiscuous parents have more sons have been replicated, or, more precisely in this case, preplicated.