Returning Soldier Effect

I have a vague memory from a long time ago. Someone told me that after wars, nature wants to replenish the killed men, and the proportion of boys among babies increases. I recently remembered this claim and looked it up. Yes, the statistical part is real: during and right after both of the World Wars, the proportion of boys increased. This effect even has its own name: the Returning Soldier Effect. Its explanation, however, is still missing in action. By the way, old observations of the sex ratio are more interesting than modern ones, as many years ago, people couldn’t use sex-selective abortions. This means that getting newer data might be useless. One interesting theory relates this effect to the frequency of sex. During and right after wars, there were more women than men, and it was easier for men to find sex partners and have a lot of sex. But why would more frequent sex for men produce more boys? This theory would work if more frequent ejaculation favored Y-bearing sperm over…