Statistical riddles in the cradle

No one knows why more boys than girls are born in the U.S. - or why the gap is lessening


July 01, 2005|By Mariana Minaya | Mariana Minaya,SUN STAFF

Expectant parents who are navigating baby-naming books and Web sites may want to take a closer look at the boys' list. Overall, the birth of a young Justin, Thomas or Scott is more likely than the birth of a girl, although the odds aren't as great as in past years.

A recent report compiled by the National Center for Health Statistics found that for the 63rd year in a row, more boys were born in the United States than girls. For every 1,000 girls, there were 1,048 boys born in 2002, the most recent year for which figures were available.

That amounts to about 94,000 more newborn boys than girls, the latest example of a longtime phenomenon that prompts scientists to offer theories but no solid explanations.

The report found that the difference between the numbers of boys and girls born is growing smaller, for reasons that also remain unknown.

The sex ratio, or number of boys born each year divided by the number of girls multiplied by 1,000, is on the decline. In 1940, there were 1,055 boys born for every 1,000 girls.

"With a decrease in the sex ratio, what you're getting is more and more equal," said Brady Hamilton, co-author of the report and statistician and demographer at the center. "Typically in the past, it has been higher."

The trends are significant, said one scientist who is familiar with the data.

"Sex ratio information has a great impact on our society," said Dr. James Zuckerman, professor of obstetrics, gynecology and reproductive biology at Harvard Medical School in Boston. "The numbers are very large, and the effect is very small and subtle, but it's quite real and statistically significant."

The decreasing sex ratio could have important social, economic and political effects in the future, he said. If there are fewer males, there may be a greater number of single-parent families, he said.

"A paper like this opens as many questions as it answers," Zuckerman said. "In the very near future some exciting things are going to come out of this."

The data have been compiled from the birth certificates filed in the United States since 1940, around the time universal reporting began.

Collecting the data may be easier than explaining the trends the authors found. Experts have puzzled over the sex ratio for centuries.

Many scientists believe the differences can be explained by events that occur during gestation, Zuckerman said.

The X and Y chromosomes determine the gender of a human during reproduction. Females require two X chromosomes; males have one of each. Some studies suggest Y-bearing sperm may have an advantage in reaching the egg, Zuckerman said, while others dispute that.

There is also debate over factors involved at subsequent stages of human development. "Is there a difference in what happens in embryos destined to be female and embryos destined to be male?" Zuckerman said.

Scientists also wonder if the sex ratio is nature's way of stabilizing populations.

Decades ago, two Harvard scientists proposed that a mother would invest energy and resources in the gender that would give the greatest reproductive success over a long period. A male's ability to generate the most offspring and grand-offspring could explain the male-biased sex ratio.

All of the potential explanations are very theoretical, Zuckerman said.

"They're very, very tough things to prove," Zuckerman said. "A lot of this is not what we call hard science."

Karen Norberg, a medical research associate at the National Bureau of Economic Research, is the author of a report that found women who cohabit with men before or during conception were more likely to have sons.

Why might that be? Dr. Steven Orzack, a research scientist at the Fresh Pond Research Institute who works with sex ratio, said there is a body of theory that suggests such women can somehow judge that a son's socialization will be more successful with a father around, making them likelier to produce male offspring.

With any of these theories, Zuckerman said, the challenge remains to find out how mothers throughout generations process that information to produce more baby boys.

"The mechanisms by which these effects might be carried out are extremely important, and give us windows of insight into how the human reproduction process and all its subtleties is carried out," he said.

Valerie Grant, a professor at the Auckland School of Medicine in New Zealand, has written that women with dominant personalities have higher levels of testosterone, and therefore have more male than female offspring.

That could explain the increase in the number of boys born compared with girls immediately after wars. She has written that women who take on dominant roles and experience greater stress when men go off to war may experience higher levels of testosterone.

The male-biased sex ratio could also be nature's way of making up for the fact that women live longer. Men have shorter life spans, perhaps because they engage in riskier behavior than women do.

In the United States, women outnumber men in older age groups. Up until age 39, there are more men than women, said T.J. Mathews, author of the report and demographer at the health statistics center.

At age 39, the situation flips.

In any case, he said, if women complain of a shortage of men they should keep in mind, "that's going to change neighborhood to neighborhood."

For every 1,000 GIRLS, there were 1,048 BOYS born in 2002

Baltimore Sun Articles
Please note the green-lined linked article text has been applied commercially without any involvement from our newsroom editors, reporters or any other editorial staff.