Gap Narrows Between Male, Female Births

April 9, 2007— -- A new study has renewed the debate among public health experts over figures that seem to show a gradual but persistent decrease in the ratio between the birth of baby boys and baby girls.

The authors of the study, published in this week's edition of the online journal Environmental Health Perspectives, point out that the proportion of baby boys being born has fallen each year in the United States and Japan since the 1970s, according to public health records in both countries.

They say this downward trend may point to the impact of environmental pollutants on male fertility and fetal development -- "a serious matter," the authors note in the study.

Critics, however, say the decrease is only slight and could indicate any number of nonenvironmental factors.

Still, study authors say the numbers should not be ignored -- particularly in light of their finding that an increasing proportion of fetal deaths are male.

Taken together, the study's authors say, the numbers add up to an overall decline of 17 males per 10,000 births in the United States and a decline of 37 males per 10,000 births in Japan since 1970.

This means, according to the authors, that since 1970, 135,000 white males in the United States and 127,000 males in Japan should have been born but were not.

Traditionally, the expected ratio of male to female births is believed to be 1.05. That is, for every 100 girls born, you would expect 105 boys to be born.

The reason for the difference, says Christopher Wills, professor of ecology, behavior and evolutionary biology at the University of California at San Diego, is that Mother Nature stacks the deck in favor of male births. Nature gives males an edge at birth because male fetuses and babies are less hardy than female fetuses and babies. So, by the time males reach the age at which they can reproduce, there should be a one-to-one ratio.

"Generally speaking, males are at a disadvantage compared to females," he says. "In general, as a species, you would want to end up with a roughly one-to-one ratio. The only way to do that is to start out with a much more skewed ratio, and that's what Mother Nature has done."

In 2002, however, this number was 104.6 for babies born in the United States.