Latitude may be one of the factors that determine the sex of your baby, according to new research. If you want a girl, head to the tropics.
A paper published in the Royal Society's Biology Letters suggest that there's evidence linking variations in the ratio of male and female births with specific latitudes.
The study is based on information gathered from more than 200 countries by the Central Intelligence Agency between 1997 and 2006 and considers variations from country to country, including socio-economic impacts. Overwhelmingly, however, the data shows that latitude was the clear determining factor.
"Globally, human sex ratios were significantly and positively correlated with latitude and its related climatic variables, but not with socio-economic status," said Dr Kristen Navara, author of the paper.
Global birth ratios average 51.5 per cent male, or 106 males for every 100 females. However, in the tropics this ratio drops to 51.1 per cent, a significant discrepancy in actual births.
Prior research has explored the social environment impacts, but Navara highlights the importance of latitude in her study, saying "this relationship emerged despite enormous lifestyle and socio-economic variation among countries and continents."
The paper adds to existing studies on the impact of temperature on birth ratios, including one in Germany and one recently completed in Finland, which offered only small scale data-sets with no tropical regions.
Although the precise causes for the variations are still unclear, Dr Navara concluded that day length and temperature had considerable influence.
"Clearly, a relationship exists between human natal sex ratios and latitudinal variables on a global scale," she said. "Ejaculate quality and fertility rate in humans vary with day length."
"The results shown here could indicate an adaptive strategy employed by humans," she added.