Contemporary selection pressures in modern societies? Which factors best explain variance in human reproduction and mating?

Abstract

Phenotypic traits in humans are under selection pressure and are still evolving, but the relative importance of these traits remains to be investigated. We therefore analyzed jointly phenotypic traits associated with number of children and having ever been married. This provides insights into the relative contribution of each trait and indicates the potential selection pressure induced by a specific trait relative to others. To shed light on potential selection on the genome level, all analyses include a multivariate polygenic risk score of general cognitive ability. We used the data from the Wisconsin Longitudinal Study (WLS), a dataset consisting of 4991 men and 5326 women almost all whites, educated at least at A-level. The focus was on the association between age, education level, wages, religious intensity, fathers' age at child's birth, ratings of facial attractiveness, number of siblings of the respondent, as well as the polygenic risk score of general cognitive ability on the following dependent variables: i) number of children, ii) ever being married, and iii) age at first birth. For each factor we additionally examined the relative contribution to the overall variance explained of the dependent variable. Having been married and, thus, mate selection, is the most important determinant for the number of children for both men and women. Wages explain most of the total variance for “ever married”, yet in different directions for men and women, as is also the case for the association between wages and number of children. In both women and men, education explains most of the variance in age at first birth, and the effect is postponing. Furthermore, although the phenotype education is negatively associated with the number of children in both sexes, this holds true for the polygenic risk score for cognitive ability only in men. In addition, in men, the polygenic risk score for cognitive ability also has a positive effect on reproduction due to its positive interaction with wages. Anyhow, with the exception of having ever been married, all other variables explain only a small proportion of the variation in fertility outcomes. Although our results are consistent with the hypothesis that there is selection pressure for rather recently arising traits as education and income, on the basis of our results we are not able to draw any final conclusion on selection.