Published on 04/15/09

Tropics produce more baby girls than anywhere else

By April Reese Sorrow

Women who want to increase their chances of giving birth to a girl should live closer to the equator, says a University of Georgia researcher, whose recent study sheds light on how temperature and day length can influence human reproduction.

Most animal sex ratios are 50:50. But that ratio can be changed by environmental factors like temperature or even by seasonal shifts, said Kristen Navara, a reproductive endocrinologist with the UGA College of Agricultural and Environmental Sciences.

Her recent research with certain hamsters and mice shows that more males are born in winter when days are shorter, and that more females are born if day length is longer. Sparked by curiosity and these findings, she wanted to see if the same held true for humans.

She dug through the CIA’s The World Factbook and other government publications to gather data on every nation with a decade of uninterrupted statistics. She gathered statistics on 202 nations. Navara then analyzed the figures based on latitude, average temperature, day length and socioeconomic status.

What her first-ever global analysis on birth sex ratio shows is that people living in areas with longer, darker winters - such as in temperate and subarctic climates - have more baby boys, accounting for 51.3 percent of births. In the tropics, 51.1 percent of births are boys. Her findings were reported in the April 1 issue of Biology Letters.

“I don’t know which cue is affecting the rates,” Navara said. “I suspect it is day length and probably melatonin.”

Melatonin is a hormone that regulates the sleep-wake cycle and the production of female reproductive hormones, Navara said. Its release varies in response to day length and season.

People living in temperate climates get fewer than eight hours of light in winter. Because of this, their bodies produce more melatonin than those living in the tropics, which has more daylight hours.

“What we might be seeing is that humans respond to a cue they were initially programmed to respond to,” she said. “Sex ratios are not necessarily tied to socioeconomic status or natural disasters, as those are always changing. There are all kinds of factors that affect the sex of a baby, but latitude does not change throughout evolutionary time.”

On average worldwide, 105 boys are born for every 100 girls. Studies show that ratio can deviate based on a large number of social, economical and physiological variables. At times of extreme environmental stress, like war for example, the birth rate of girls exceeds that of boys.

The increased percentage of girl babies in the tropics may seem small. But it can translate into a lot of babies.

In Central African Republic, for example, 51 percent of babies born are girls, the only country in the world to produce more girls than boys. In 2006, this translated into 1,400 fewer boys than if the ratio was 50:50.

Other tropical countries with lower male birth rates included Grenada with 50.2 percent, Mauritius with 50.3 percent and the Bahamas with 50.5 percent.

“Of the 20 countries with the lowest ratios, 18 were at tropical latitudes,” Navara said. “We found that the difference was independent of other cultural variables, including socioeconomic status. It was an over-arching pattern and this effect remained despite enormous cultural variations between the countries we looked at.”

The biological trend works independently of cultural factors. In some societies in Asia and Africa, for instance, baby boys are favored over girls and the rise in selective abortions and infanticide has skewed the overall sex ratio in favor of males.

“I eliminated all Asian and African countries during a second round of analyses to get rid of any confound associated with sex-specific abortion,” she said. “The trend of women giving birth to girls the nearer they are to the equator was still significant.”

Navara is currently studying the cues that affect the sex ratios in poultry.

April R. Sorrow is a science writer with the University of Georgia Public Affairs Office.