By Martin Fieder
Evolutionary biology has always been interested in the question, which individual traits influence how many children we have, and how much each of these traits contributes to it. As nearly every trait has to some extent a genetical basis, together with the progeny also the genes that are associated with any favorable trait are spreading – in evolutionary terms “the trait is under positive selection”.
It is meanwhile known that several traits influence how many children a person has, at which age a person becomes mother or father, or what the chances are that a person finds a mate. It is also established that the effects of those traits may differ whether the person is a woman or a man. Moreover, for some of these traits their genetical basis in terms of so-called polygenic risk scores has been determined, which enables us to estimate the liability of an individual for a certain trait. What has been largely neglected so far is the question, how much each trait contributes (in statistical terms how much variance is explained) to the number of children or the probability of finding a mate/being ever married. We therefore aimed to explore that issue and analyzed how much each single trait contributes to the individual variance of number of children and ever being married. As we used the Wisconsin Longitudinal study (WLS) for that analysis, ever being married is a rather good proxy for mating in general because participants in that data set are born between 1937 and 1940, as marriage was very common at that time in the US.
Among these analyzed traits are religious intensity (as it is known for its pro-fertile effect in both men and women), facial attractiveness as well as social status because social status in terms of income is also known to foster number of children in men though not in women. Our analyses showed that among men, this positive effect of social status on a man’s number of children can simply be explained by mating. Being married was by far the most important factor of having children in this data set. And the factor that contributed most to the fact, whether or not a person was ever married or not, was income (~18% in men and ~7% in women). The direction of that effect, however, differed between men and women. In men, higher wages were associated with a higher chance of ever being married. Accordingly, female preference for high social status in terms of high income predicted a man’s chances of marriage and hence, his number of children. In women, on the other hand, higher wages were associated with a lower chance of being married. We do not have a final explanation for the negative effect of income on ever being married in women, but likely the problem of combining family issues and career plays a role.
The effect of education, another indicator of social status, on marriage and reproduction was less straightforward. In both men and women of our data set, higher education predicted a lower number of children, which is in line with the literature and, due to postponing of reproduction, is particularly expected in women. Interestingly, in men but not in women, the genetic predisposition towards higher education (polygenetic score for general cognitive ability) had a small positive effect on the number of children. A more detailed analysis revealed, that in men, a higher predisposition for education/general cognitive ability was associated with higher income, which in turn predicted a higher number of children.
“Our data show that in men, social status in terms of income has by far the strongest effect on marriage and as a result on reproduction as well. This is a finding which is clearly in line with evolutionary assumptions, predicting that women should prefer men of higher social status for their own and the sake of their progeny”
The effect of all other analyzed traits on the number of children or ever being married was much weaker. Only facial attractiveness explained 6% of ever being married in women and 2% in men, which, however, is still lower than expected as facial attractiveness has always been reported to be an important determinant for mating.
Our data show that in men, social status in terms of income has by far the strongest effect on marriage and as a result on reproduction as well. This is a finding which is clearly in line with evolutionary assumptions, predicting that women should prefer men of higher social status for their own and the sake of their progeny. However, as income is a very recent indicator of social status we are not able to make any final conclusions on ongoing selection pressures in modern societies. On the basis of our data, we can just argue that men in modern societies seem to be under pressure to “earn money” if they aim to find a mate and to have children.