More Females are Born in Times of Stress

A stress-related decline in the sex ratio is no anomaly, but a biological mechanism that evolved to keep the species going, according to Ralph Catalano, professor of public health at the University of California, Berkeley. “From an evolutionary perspective, in stressful eras it’s a smarter bet to have a female child than a male child. A daughter is more apt to provide you with grandchildren,” Catalano tells me. In hard times, he explains, weak males are less likely to survive and to reproduce...

 Environmental Influence on Gender

There are many well-established natural factors that bias the sex ratio of human offspring, proving that it is at least possible. The most famous is the returning-soldier effect. During and immediately after major wars, more sons are born than usual in the belligerent countries as if to replace the men that died. (This would make little sense; the men born after wars will mate with their contemporaries, not with those widowed by the war). Older fathers are more likely to have girls, but older...

 The Give and Take of Gender on Evolution

Trivers and Willard realized that the same general principle of sex allocation, which determines the gender of nematodes and fish, applies even to those creatures that cannot change sex but that take care of their young. They predicted that animals would be found to have some systematic control over the sex ratio of their own young. Think of it as a competition to have the most grandchildren. If males are polygamous, a successful son can give you far more grandchildren than a successful daugh...