It points to big differences in the proportions of male to female births between tropical latitudes and temperate and sub-arctic latitudes.

This gap remains even when local cultural and social preferences -- such as the preferences for males in India and China, resulting in the abortion of female foetuses -- are taken into account.

University of Georgia endocrinologist Kristen Navara looked at official data collected from 202 countries over a decade, from 1997-2006, and published in the Central Intelligence Agency (CIA) World Factbook.

Averaged around the world, there were 51.3 percent male births to 48.7 female births, she found.

But this average masked big differences according to latitude: in tropical latitudes, male dominance fell to 51.1 percent of births. In tropical sub-Saharan Africa, it was just 50.8 percent.

"Significantly more females were produced at tropical latitudes," she told AFP. "This relationship emerged despite enormous lifestyle and socio-economic variation among countries and continents."

Navara, whose paper appears in the British journal Biology Letters, said more research is needed to explain the puzzle.

She speculated that human gametes -- sperm and eggs -- may be affected by ambient light and temperature, and this could exert a bias in favour of one gender or another.

Previous studies in small mammals (Siberian hamsters, house mice and meadow voles) have discovered that these animals produce more males during the winter months or when daylight hours are fewer.

In China, the pro-male bias was 52.8 percent while in India it was 51.2 percent, slightly below the global mean, Navara said.

(c) 2009 AFP