By Alison McCook

NEW YORK (Reuters Health) - People tend to give birth to higher numbers of boys in certain regions of the world, a curious trend that appears to be linked to latitude, new research reports.

The investigators found that, in Europe, more boys are born in southern latitudes, while in North America, more boys are reported in the northern region of the continent.

"We are unable to explain these findings, which do not support a temperature-related effect," the researchers noted in the April 27th issue of the British Medical Journal.

The authors, led by Victor Grech of St. Luke's Hospital in Malta, analyzed 50 years of births in North American and European countries reported to the World Health Organization (news - web sites). They examined records from more than 550 million births.

Overall, humans tend to give birth to slightly higher numbers of boys than girls, with 51.5 boys born for every 48.5 girls.

However, past research has suggested that, in the last 30 to 50 years, male births have been declining in relation to female births.

In Europe, the researchers found that all European countries reported sex ratios less than the expected 0.515, for a total deficit of 238,693 male births. However, southern countries, including Spain, Greece, and Malta, reported more boys than central Europe or Nordic countries like Sweden or Iceland.

In North America, Grech and his colleagues found that generally more boys were born in Canada and the US than in Mexico. All countries reported sex ratios slightly less than the expected 0.515.

In an interview with Reuters Health, Grech said he had "not a clue" as to any biological explanations for these strange findings.

"We have no explanation but it can't be a coincidence. The odds against these results occurring by chance alone are millions to one," he said.

Grech said he and his colleagues decided to investigate whether a global skew in sex ratios existed after they found that, over a 5-year period in Europe, fewer boys were born in higher than in lower latitudes.

Various theories have been offered to account for the overall decline in the number of expected male births, including the fact that the male fetus is more susceptible to environmental damage, and is generally more fragile than the female fetus.

SOURCE: British Journal of Medicine 2002;324:1010-1011.

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