Slow and Steady Wins the Race: K Positively Predicts Fertility in the USA and Sweden

Nothing is presently known about the relationship between individual differences in fertility and life history (LH) speed, as measured by the K-Factor. To examine this relationship, the correlation between LH speed and the number of children was examined in two, large samples (MIDUS II and the Swedish STAGE dataset). Their association was positive and statistically significant in both cross-national samples. The association was robust with respect to statistically controlling for participant age. Nested model comparison of a Model looking only at linear effects with a second Model incorporating a quadratic term did not improve model fit in any instance, suggesting directional selection for slower LH, The heritability of the indicators comprising the K-Factor positively moderated the strength of selection, while K-Factor loading weakly negatively moderated selection strength, suggesting that K-Factor variance, as a multivariate latent construct, is not the primary target of selection. These results are consistent with fertility intentions data indicating positive correlations between slower LH and desired numbers of children. In modern environments, higher mating effort does not appear to result in more offspring, likely because of strategic interference suppressing the fertility of those with fast LH, stemming from influences that may be either endogenous (i.e., contraceptive usage) or exogenous (i.e., the presence of laws, such as alimony) to the individual.