Unnatural Selection 1: Sex Selection Begins With the Second Pregnancy

This week I will be blogging Mara Hvistendahl's Unnatural Selection: Choosing Boys Over Girls, and the Consequences of a World Full of Men.

Hvistendahl is trying to figure out why, exactly, there have been so many fewer female births in some Asian countries over the past generation.

She thinks that smaller family size in developing countries means that the risks of having no boys by natural means goes up from about one in 10 if you have three children, to one in four if you have only two. Thus, richer, smaller families are going with nature in first births, but in each successive pregnancy are more likely to abort a girl, but keep a boy.

She cites as evidence the sex ratio of boys to girls at birth in South Korea in 1989. The natural ratio is about 105 boys for every 100 girls.

First birth: 104/100
Second birth: 113/100
Third birth: 185/100
Fourth birth: 206/100