Satoshi Kanazawa

The Scientific Fundamentalist

Sex

Boy or girl? What determines the sex of your child? IV

Sexually promiscuous parents have more sons.

Posted Feb 23, 2009

Sexually promiscuous parents have more sons

The generalized Trivers-Willard hypothesis suggests that any heritable trait of a parent which is disproportionately beneficial to either sons or daughters can bias the sex of the offspring in the direction that benefits the parent’s reproductive success.  It has so far been shown that engineers and scientists are more likely to have sons while nurses and social workers are more likely to have daughters; big and tall parents are more likely to have sons; violent parents are more likely to have sons; and beautiful parents are more likely to have daughters.  Another factor which can subtly influence the sex of the offspring is the parent’s sociosexual orientation, how sexually promiscuous the parent is.

In our article forthcoming in the Annals of Human Biology, Péter Apari of Eötvös University and I extend the generalized Trivers-Willard hypothesis to sociosexual orientation.  Compared to those with restricted sociosexual orientation, individuals with unrestricted sociosexual orientation are more likely to:  1) engage in sex at an earlier point in their relationships; 2) engage in sex with more than one partner at a time; and 3) be involved in sexual relationships characterized by less investment, commitment, love, and dependency.  While men in general are more unrestricted in sociosexuality than women, within-sex variance in sociosexual orientation is much larger than between-sex variance.

Due to the sexual asymmetry in reproductive biology, unrestricted sociosexual orientation could potentially and dramatically increase men’s reproductive success, while it is likely to decrease women’s.  Men with unrestricted sociosexual orientation can impregnate a large number of women simultaneously, and, even without male parental investment, some of the resultant children are likely to survive to sexual maturity.  In contrast, women with unrestricted sociosexual orientations can have no more children than their restricted counterparts with one regular sex partner, and are unlikely successfully to secure male parental investment from the fathers of their children because none of the men can be reasonably sure of their paternity.  Further, women in committed  relationships who are sociosexually unrestricted may incur significant somatic costs of spousal and partner violence, because male sexual jealousy is often triggered by real or imagined sexual infidelity, and many mated men often use violence as a tactic for mate retention.

There is evidence to suggest that sociosexual orientation as well as risk of divorce, which often results from unrestricted sociosexual orientation, are highly heritable.  A study of a large sample of Australian twins, conducted by the behavior geneticist J. Michael Bailey, whom we have encountered before, and colleagues, shows that nearly half of the variance in sociosexuality is attributable to genes.  In other words, Madonnas beget Madonnas, and whores beget whores.  As I explain in an earlier post, the risk of divorce is another heritable trait which follows the 50-0-50 rule:  about 50% of the variance is heritable, about 0% is due to shared family environment, and about 50% is due to unshared environment.  So roughly half of the variance in the risk of divorce, as well as sociosexual orientation, is heritable.

The logic of the generalized Trivers-Willard hypothesis would therefore suggest that individuals with unrestricted sociosexual orientation (“whores”) are more likely to have sons than those with restricted sociosexual orientation (“Madonnas”).  Our analyses of two large nationally representative samples, from the General Social Survey in 1994 and the National Longitudinal Study of Adolescent Health, confirm this prediction.  Controlling for a large number of social demographic factors that might be expected independently to influence offspring sex ratios, unrestricted sociosexual orientation significantly increases the odds that the first child is a boy.  One standard deviation increase in the unrestrictedness of sociosexual orientation increases the odds of having a son by 12-19%.

It therefore appears that sociosexual orientation, whether you are relatively restricted or unrestricted in your sexual behavior, is yet another factor that might subtly and slightly influence the sex of your child, along with your brain types, body size, tendency toward violence, and physical attractiveness.