Human genealogy reveals a selective advantage to moderate fecundity.

Life-history theory suggests that the level of fecundity of each organism reflects the effect of the trade-off between the quantity and quality of offspring on its long-run reproductive success. The present research provides evidence that moderate fecundity was conducive to long-run reproductive success in humans. Using a reconstructed genealogy for nearly half a million individuals in Quebec during the 1608-1800 period, the study establishes that, while high fecundity was associated with a larger number of children, perhaps paradoxically, moderate fecundity maximized the number of descendants after several generations. Moreover, the analysis further suggests that evolutionary forces decreased the level of fecundity in the population over this period, consistent with an additional finding that the level of fecundity that maximized long-run reproductive success was above the population mean. The research identifies several mechanisms that contributed to the importance of moderate fecundity for long-run reproductive success. It suggests that, while individuals with lower fecundity had fewer children, the observed hump-shaped effect of fecundity on long-run reproductive success reflects the beneficial effects of lower fecundity on various measures of child quality, such as marriageability and literacy, and thus on the reproductive success of each child.