After war, there is a baby boy boom in the nations who faced the war. There are many theories for why more boys are born after wars. One explanation is that soldiers who return from war have a lot of sex.
Another theoretical explanation is that it is nature’s way of replenishing boys for the men lost in battle. These theories are inaccurate, though. We know that the male sperm decides the sex of the unborn child.
A researcher from Newcastle University found that the answer is heredity. Men who have more brothers than sisters are more likely to have sons. Family trees that show more males born will likely continue to produce more boys from the men in the family tree.
This is not true for the women in the family tree, seeing as how the sex of a child is decided by the male sperm. This affects baby boy booms after war, because men are more likely to be soldiers. They return, and often multiple sons go to war, and heredity says they too will produce more sons.