WHERE HAVE ALL THE BOYS GONE?

Despite their macho swagger, males are the more fragile sex of the human species. Male fetuses are less likely than females to come to term: although 125 males are conceived for every 100 females, only about 105 boys are born for every 100 girls. In the first half of this century, improvements in prenatal care reduced the number of miscarriages and stillbirths and hence increased the proportion of baby boys in most industrial countries. But since 1970 the trend has reversed: in the U.S., Canada and several European countries, the percentage of male births has slowly and mysteriously declined.

So far the decrease has not been alarmingly large. In the U.S. in 1970, 51.3 percent of all newborns were boys; by 1990, this figure had slipped to 51.2 percent. But in Canada the decline has been more than twice as great, and similar long-term drops have been reported in the Netherlands and Scandinavia. The U.S. and Canadian data were compiled by Bruce B. Allan, an obstetrician-gynecologist at Foothills Hospital in Calgary, Alberta. Allan claims the widespread nature of the decline suggests that it is more than a statistical fluctuation. "We can’t deny that the percentage of boys is falling," Allan says. "But the question is, Why?" Demographic factors may be playing a role. Different races have slightly different birth ratios: blacks tend to have fewer boys than whites, whereas Asians have fewer girls. (These differences have been observed worldwide.) The parents’ ages may also influence the gender of their offspring; studies have shown that older fathers sire fewer sons than young dads. But Allan found that demographic changes in the Canadian population between 1970 and 1990 could not account for the decline in the percentage of baby boys there. Some researchers believe pollution may be the culprit. A recent article in the Journal of the American Medical Association notes that high exposures to certain pesticides may disrupt a father’s ability to produce sperm cells with Y chromosomes_the gametes that beget boys. Other toxins may interfere with prenatal development, causing a disproportionate number of miscarriages among the frailer male embryos. (XY embryos require hormonal stimulation to produce masculine genitalia, which may make the unborn males more vulnerable to hazardous chemicals.)

Perhaps the most striking example of a lopsided birth ratio occurred in Seveso, Italy, where a chemical plant explosion in 1976 released a cloud of dioxin into the atmosphere. Of the 74 children born to the most highly exposed adults from 1977 to 1984, only 35 percent were boys. And the nine sets of parents with the highest levels of dioxin in their blood had no boys at all.