So, just what is the "sex ratio"?
For every 100 baby girls, there are 105 baby boys. This is a fact.
Why more boys? Nobody knows! And it's a real puzzle because...
- Other animal species have half male and half female offspring.
- Men produce equal numbers of x-chromosome sperms (which make girls) and y-chromosome sperms (which make boys).
Testosterone and Conception
Women have only one tenth the amount of testosterone that men do. But even these tiny amounts may somehow influence whether a woman conceives a boy or a girl. If the woman has more than average testosterone, the hormonal environment may make it easier to conceive a male. Or if she has less than average testosterone it may be that her body will be better suited, hormonally, to nurturing a female embryo.
Dominance and Testosterone
In both humans and animals, testosterone has been shown to be related to dominance. The research also shows how testosterone varies over time within individuals, according to events in the environment. There is also evidence to suggest that testosterone in women rises during periods of chronic stress - the sort of situation that arises in time of war, famine or disease. There are several documented examples of more boys being born following such times, provided conditions have improved during pregnancy.
Although the sex ratio averages out everywhere to 105 boys for every 100 girls, researchers have found that men in some occupational groups have a tendency to have more girls. Some studies have found that airline pilots, astronauts and deep sea divers have more girls. Recently, workers in timber mills have been shown to have more daughters. There are likely to be at least two reasons for this. First, it may be that men in these occupations tend to marry less-dominant women, who are more likely to conceive girls. Secondly, it may have something to do with the nature of some of these occupations. It is liekly that both are relevant.
Dominant female deer have more male offspring than non-dominant deer; far more than would be expected by chance. A famous study describing this phenomenon was published in the top science journal Nature in 1984. Since then there have been more than forty animal studies published in reputable scientific journals documenting unusual sex ratios. For most animals living in the wild - different kinds of monkeys, wild horses and even whales - dominant mothers have more male offspring. When the animals were kept in captivity, however, the sex ratios sometimes ran in the opposite direction and the dominant females had more female offspring. We think there is a good explanation for this because caging is likely to have an effect on both social hierarchies and hormones.