More boys than girls are born all over the world, but a study has found that the closer people live to the equator, the smaller the difference becomes. No one knows why.

The skewed sex ratio at birth has been known for more than 100 years, and researchers have found a large variety of social, economic and biological factors that correlate with it -- war, economic stress, age, diet, infanticide and more. Teasing out the contribution of any single cultural or political variable has proved a complicated exercise.

But researcher Kristen J. Navara of the University of Georgia performed a statistical analysis that found the effect of latitude persisted across wide variations in lifestyle and socioeconomic status. Her report was published online April 1 in Biology Letters. The correlation with latitude was unchanged even after excluding data from countries that might have been skewed by abortion or the killing of baby girls.

It could be that there is some survival value in producing more girls in warmer regions. Could the quality of sperm at different temperatures cause the variation at the moment of conception?

Or is there some event during gestation at warmer temperatures that causes more male fetuses, or fewer female, to spontaneously abort? Hamsters, mice and meadow voles also produce more male offspring during shorter days or colder weather, but the reasons in these animals are just as mysterious as they are in humans. Said Navara: "What's interesting is that we may be seeing something that attests to our animal ancestry."

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