Sociosexually unrestricted parents have more sons: A further application of the generalized Trivers–Willard hypothesis (gTWH)

Abstract

Background: The generalized Trivers–Willard hypothesis (gTWH) proposes that parents who possess any heritable trait which increases male reproductive success at a greater rate than female reproductive success in a given environment will have a higher-than-expected offspring sex ratio, and parents who possess any heritable trait which increases the female reproductive success at a greater rate than male reproductive success in a given environment will have a lower-than-expected offspring sex ratio.

Aim: One heritable trait which increases the reproductive success of sons much more than that of daughters is unrestricted sociosexual orientation. We therefore predict that parents with unrestricted sociosexual orientation (measured by the number of sexual partners, frequency of sex, and attitudes toward relationship commitment and sexual exclusivity) have a higher-than-expected offspring sex ratio (more sons).

Subjects and method: We analyse the US General Social Surveys and the National Longitudinal Study of Adolescent Health (Add Health), both with large nationally representative samples.

Results: Our analyses support the prediction from the gTWH.

Conclusion: One standard deviation increase in unrestrictedness of sociosexual orientation increases the odds of having a son by 12–19% in the representative American samples.

• Evolutionary psychology
• generalized Trivers–Willard hypothesis (gTWH)
• offspring sex ratio
• sociosexual orientation

Notes

Notes

1. It is important to point out that the gTWH does not predict that parents with a given set of heritable (and immutable) traits will produce offspring of only one sex or the other. Assume that the sex of a child is determined by a toss of an imaginary coin, and the ‘normal’ or baseline parent has an ‘unbiased’ coin that comes up ‘boy’ 51.22% of the time and comes up ‘girl’ 48.78% of the time (to reflect the ‘normal’ secondary sex ratio of 105 boys to 100 girls). If a parent possesses some heritable trait that increases male reproductive success, then the coin becomes ‘biased’ in favour of sons and now comes up ‘boy,’ say, 55% of the time. Such a parent is therefore significantly more likely to have a boy than the ‘normal’ or baseline parent; however, it does not mean that such a parent will only have boys. It is still possible (albeit less likely than for others) for a parent with a probability of having a boy p=0.55 to have a girl, or even three girls in a row.

2. One interesting point to note on the side is that the correlation between the lifetime number of heterosexual partners and the lifetime number of homosexual partners is significantly positive. The correlation is larger among women (r=0.2642, p<0.0001, n=9715) than among men (r=0.0518, p<0.0001, n=7546). The positive correlations are not created entirely by outliers. If we limit the sample only to those who have had fewer than 100 heterosexual and 100 homosexual partners, the correlations actually increase both among women (r=0.4162, p<0.0001, n=9703) and men (r=0.2257, p<0.0001, n=7430). The correlation becomes negative among men only if we limit the sample to those who have had fewer than 50 partners of each kind (r=–0.0380, p<0.01, n=7066). It never becomes negative among women, even when we limit the sample to those who have had fewer than 10 partners of either kind (r=0.0107, NS, n=8877).

3. Preliminary analysis shows that, unlike the GSS data analysed above, the frequency of sex in the last 12 months in the Add Health data is not correlated with these five measures of sociosexual orientation and does not load on the same factor; it instead extracts its own factor. We have therefore excluded the frequency of sex as an indicator of sociosexual orientation here.

4. Among male Add Health respondents, physical attractiveness is positively correlated with behavioural sociosexual orientation (more attractive men are more unrestricted, consistent with Gangestad and Simpson (2000)) (r = 0.0626, p<0.00 001, n=6900), but is negatively correlated with attitudinal sociosexual orientation (more attractive men are more restricted) (r=–0.0674, p<0.00 001, n=6900). Among women, physical attractiveness is not correlated with behavioural sociosexual orientation (r=0.0046, NS, n=7783) and is negatively correlated with attitudinal sociosexual orientation (r=–0.0498, p<0.0001, n=7783).