People who live in the tropics have more baby girls compared to people living in other parts of the world. It could be the result of hotter weather or longer days, which might change miscarriage rates and sperm quality.

Research suggests the female fetus is less fragile than the male fetus. The male fetus is therefore more susceptible to the effects of the environment. At times of extreme environmental stress, including war, the birth rate of girls outstrips that of boys.

Experts have long suspected that latitude could also have an effect. A new study has now shown that countries closer to the equator produced a significantly lower percentage of boys annually than those at temperate latitudes 51.1 percent versus 51.3 percent.