| Tropics are a boon for baby girls|
|Couples hoping for a baby girl might be best advised to live in Africa, if a new study is correct.|
Researchers at the University of Georgia have found a difference in the proportions of male to female births between tropical, 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.
The study appears in the Royal Society's journal Biology Letters.
Study lead author, endocrinologist Dr 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% male births to 48.7% births, she found.
But this average masked big differences according to latitude: in tropical latitudes, male dominance fell to 51.1% of births. In tropical sub-Saharan Africa, it was just 50.8%.
"Significantly more females were produced at tropical latitudes," says Navara. "This relationship emerged despite enormous lifestyle and socio-economic variation among countries and continents."
Navara says more research is needed to explain the puzzle.
She speculates 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.
Navara adds that in China, the pro-male bias was 52.8% while in India it was 51.2%, slightly below the global mean.
In 2000, a European study suggested people in warmer climates, were more likely to produce male offspring.
The researchers analysed births in Mediterranean, Central European and Scandinavian countries and found that families living in the more chilly regions had boys less often.
Previous studies in small mammals, such as 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.