How is it that women don’t have the same number of children as men?

Fertility is usually expressed as the average number of children per woman, while men’s fertility is measured less often. In fact, the two sexes do not have the same number of children. How is it that, pour a given number of children, male fertility can differ from female fertility when each child can have only one biological father and one biological mother?

The reason is that the total numbers of men and women differ: according to the natural sex ratio, 105 boys are born for 100 girls. If we take those figures and posit that that generation will later have a total of 210 children, then completed fertility as it concerns boys who have become fathers in their turn will come to an average of 2.0 children (210/105) whereas for girls who have become mothers it will come to an average of 2.1 children (210/100).

But we also need to remember that men’s mortality exceeds that of women, leading to a reversal of the sex ratio in adulthood. And that reversal is aggravated by war, which can decimate birth cohorts of young male soldiers. One effect of this is to skew the “marriage market” as there would then be more women than men. And in that case, men’s fertility would exceed women’s. This explains why the completed fertility of men born in France in 1895 (and who were therefore age 20 at the start of World War I) came to 2.25 children on average whereas women’s completed fertility stood at 1.98—a gap of 10%. Conversely, massively male immigration reverses that gap, which is what has happened in Persian Gulf countries.

In France today, men and women have about the same number of children. But that is not the case in sub-Saharan Africa, where husbands are much older than their wives and consequently are not as numerous. In several countries in the Sahelian region, recent calculations show that average male fertility is much higher than average female fertility.