The most accepted theory about it among mammals states that the sexual proportion of individuals is a result of the animals generating more descendants of the particular gender which is more likely to produce offspring. Got it? Well, you can smell a little Natural Selection there, if you look close enough. But what about us, humans?
According to Valerie, the mothers are responsible for the choice and the hormone testosterone has a decisive role in the gender determination of the babies. She believes that the action of the hormone on the ovarian follicle may result in the formation of eggs that are predisposed to receive X or Y sperm cells.
One example of such control can be observed in the proportion of births of boys and girls after the world wars. Boys exceeded the historic averages in more than 20%. That may caused by the stress the women were exposed to during the war. Because of it, Valerie says, they were more likely to produce sons. Nowadays, the birth proportion of boys and girls is 106 for 100, respectively.