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Why are More Boys than Girls Being Born?

94,232 more boys than girls were born in 2002. This is the latest finding of the Centers for Disease Control and Prevention's (CDC) "Trend Analysis of the Sex Ratio at Birth in the United States," which measured the sex ratio at birth for more than the past 60 years.

If that statistic surprised you, you'll be even more surprised to learn that an average of 91,685 more male babies have been born each year than females from 1940 to 2002. That means a total of 5,776,130 more boys were born during that period.

Even though more boys are born, some say male births are actually decreasing.

The CDC pointed out a few other interesting findings on the ratio of boys to girls, too:

Older Moms and Teen Moms: Women in their 40s had the lowest ratio of boys to girls while late-teen moms had the highest.

Number of children: The more children a woman has, the more likely she'll have an equal number of girls and boys

Ethnicity: Chinese women had the highest ratio of boys to girls, followed by Filipino women, while American Indian moms had the lowest.

Other factors that may impact the ratio include older fathers, stress, environmental toxins and lower maternal weight, says the CDC.

During development, boys are more fragile than girls.

What's the Norm?

Although these statistics may seem alarming at first glance, they are well within the "normal" worldwide ratios, even at the extremes. Globally, there are about 105-107 boys born for every 100 girls.

At the highest sex birth ratio in the United States, which occurred in 1946, there were about 105.9 boys born per 100 girls; and at the lowest sex birth ratio (in 2001), there were about 104.6 boys born per 100 girls. There were 104.8 boys born in 2000 for every 100 girls.

For comparison, in China, where couples are only allowed to have one child, the gender balance has become alarmingly skewed and millions of men may be forced to remain bachelors because of the shortage of women.

"This is a seriously dangerous ratio," said Ren Yuling, a delegate to the Chinese People's Political Consultative Committee. "The numbers mean that some people will never have their needs for a spouse met, so they move into dangerous territory."

Just what is China's sex birth ratio? In 2000, there were 116.9 boys born for every 100 girls.

Why are more boys naturally born than girls? The New York Times offered some clues in an article called, "The Weaker Sex." Although more boys are born, it appears they may be more susceptible to a host of diseases, injuries and more, and soon women actually outnumber men. Here are just a few of their findings:

Facts on Females Living Longer & Stronger

  • Male births slightly outnumber female births, but boys have a higher death rate if born premature: 22 percent compared with 15 percent for girls.

  • Overall, more newborn males die than females (5 to 4).

  • Sudden infant death syndrome is one and a half times more common in boys than girls.
  • As teenagers, boys die at twice the rate of girls.

  • Men have fewer infection-fighting T-cells and are thought to have weaker immune systems than women.

  • By the age of 36, women outnumber men.

  • Stroke, cancer, diabetes, heart disease and accidents--all among the top causes of death--kill men at a higher rate than women.

  • American men typically die almost six years before women do.

  • By the age of 100, women outnumber men eight to one.


During fetal development "the male sex is clearly the more fragile one," said Bruce B. Allan, an obstetrician-gynecologist in Calgary, Alberta Canada.

Between 1970 and 1990, the proportion of male births decreased 1 percent.

Is the Number of Boys Actually DECREASING?

The CDC study above doesn't touch on another CDC finding, this one released in 2002, that birth rates overall had hit an all-time low. The birth rate fell to 13.9 per 1,000 persons in 2002, down from 14.1 per 1,000 in 2001. This is a full 17 percent decrease in births from 1990, when rates reached 16.7 per 1,000 people.

Take that finding, coupled with the fact that a study by Allan and his colleagues found that, from 1930 to 1990, the male sex ratio had dropped by about 1 boy per 1,000 births. Though older parents, fertility drugs and certain diseases have all been linked to an increasing proportion of female births, Michelle B. Gottlieb of the World Resources Institute (WRI), says the findings "suggest that avoidable, [likely] environmental, factors may be playing a role."

Pollutants that mimic human hormones, including dioxin have become ubiquitous in the environment and may be affecting births. For instance, in 1976, an industrial accident released high levels of dioxin into the environment near Seveso, Italy. Of the nine couples who received the highest exposure, 12 daughters were born--and no sons. Among four couples who had lower blood levels of dioxin, "The male-female ratio approached normal," said Larry L. Needham of the Atlanta CDC.

Devra Lee Davis, a program director at WRI believes that the declining male birth ratio "should be viewed as a sentinel health event," an indicator that environmental hazards difficult to detect by other means do exist and should be dealt with.

But there are other theories too. A study in the journal of the American Medical Association found that high exposures to certain pesticides could interfere with a father's ability to produce sperm cells with Y chromosomes--the kind needed for boys.

And, another study found that women who eat PCB-contaminated fish (PCBs, or polychlorinated biphenyls, can mimic female sex hormones) are less likely to give birth to boys. The women who were exposed to the highest levels of PCBs (just by eating contaminated fish) had about 50 percent fewer boys compared with women who had been exposed to the lowest levels.

So even though over 94, 000 more boys than girls are born in the United States each year, it seems it's the boys we need to worry about--in the two decades between 1970 and 1990, the proportion of male births went down an entire 1 percent worldwide.

Recommended Reading

Organophosphates: What You Don't Know Can Indeed Hurt You


National Center for Health Statistics June 14, 2005

USA Today: China Thrown Off Balance as Boys Outnumber Girls

New York Times: The Weaker Sex

Science News Online: Why are Boys Birth Rates Falling?

Where Have All the Boys Gone?

Polluted Fish Reduce Male Birth Rate

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