Intelligence and family size: A paradox resolved

Abstract

IQ data was studied for over a thousand families for both parents and offspring, and also for the non‐reproducing siblings of parents. The inclusion of these siblings allowed the authors to resolve the paradox presented by the failure of the general intelligence level to decline in accord with the large negative correlation (‐0.30) between intelligence and number of children in the family. When the non‐reproducing siblings of the parents are included, the negative correlation disappears. The higher reproductive rate of individuals in the lower IQ groups who are parents is offset by the larger proportion of their siblings who never marry or who fail to reproduce when married. Thus, the IQ level of the whole population should remain relatively static from one generation to the next, or at least not drop rapidly.