Dutch family archives hold marriage records going back to 1600. From these records, like entries in an old ledger, you can extract two columns: how many children a person had and how many years they lived. Evolutionary biologists have been arguing for half a century over whether these columns are connected. According to one theory, life is like a balance scale with two pans: put offspring on one pan, and the other pan, longevity, rises, because the body's resources are not elastic.
A new study tested this idea on nearly three hundred thousand Dutch people born between 1850 and 1915. The scientists compared relatives to separate the influence of genes from that of the environment. It turned out that the genes responsible for the number of children barely overlap with the genes responsible for lifespan. In women the link is weakly negative; in men there is none at all.
But another link proved strong: the age at first childbirth. People whose genes predisposed them to later parenthood lived longer. This connection appears in both sexes and does not depend on how many children were born in total. It seems the point is not that early childbirth drains the body, but that the same genes govern both the pace of life and its overall length.
The scale that the old theory talked about turned out not to be so sensitive after all. The weights of heredity on the offspring pan barely tip the longevity pan. But if a person is naturally inclined to postpone parenthood, their internal clock may tick more slowly. This does not refute evolutionary logic, but it forces us to look for subtler mechanisms than a simple reallocation of resources.
Data on people born in the 19th century bring us back to the same question with which the family archives began: how much does a child cost? The answer given by genes sounds surprising: almost nothing, if you count in years of life. What matters more is not the number of children but the age at which a person first becomes a parent. That age, like the hand of a clock, points to the overall pace of life set by heredity.
In the studied population, the heritability of number of children in men was 2.5 times higher than in women, which may reflect a greater influence of a man's hereditary qualities (e.g., resources or social status) on his reproductive success.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.