Science & Technology: Where the Boys Aren’t

Issue Date: 
April 29, 2007

University of Pittsburgh study notes decline in male births in the United States, Japan

During the past 30 years, the number of male births has decreased each year in the United States and Japan, according to a Pitt-led study published in the April 9 online edition of Environmental Health Perspectives.

In a review of all births in both countries, the study found that significantly fewer boys than girls were born and that an increasing proportion of fetuses that died were male. The researchers noted that the decline in births is equivalent to 135,000 fewer White males in the United States and 127,000 fewer males in Japan over the past three decades. Environmental factors may help to explain these trends, they said.

“The pattern of decline in the ratio of male-to-female births remains largely unexplained,” said Devra Lee Davis, lead investigator of the study, professor of epidemiology in Pitt’s Graduate School of Public Health, and director of the University of Pittsburgh Cancer Institute’s Center for Environmental Oncology. “We know that men who work with some solvents, metals, and pesticides father fewer boys. We also know that nutritional factors, physical health, and chemical exposures of pregnant women affect their ability to have children and the health of their offspring. We suspect that some combination of these factors, along with the older ages of parents, may account for decreasing male births.”

Davis explained that environmental factors, such as prenatal exposure to endocrine-disrupting environmental pollutants, may impact the SRY gene—a gene on the Y chromosome that determines the sex of a fertilized egg. Other factors that also may affect the viability of a male fetus include the parents’ weight, nutrition, and the use of alcohol and drugs.

In the study, Davis and her colleagues reported 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. They also found that while fetal death rates have generally decreased, the proportion of male fetuses that die has continued to increase. In Japan, among the fetuses that die, two-thirds are male, up from just over half in 1970.

The study also examined the ratio of African American male-to-female births to that of Whites in the U.S. The researchers found that while the number of African American male births has increased modestly over time, the ratio of male-to-female births for African Americans remains lower than that of Whites. In addition, they noted that African Americans have a higher fetal mortality rate overall and a higher proportion of male fetuses that die.

“These results are not surprising, since the Black-White ratio in terms of infant mortality has remained the same for almost 100 years,” said study coinvestigator Lovell A. Jones, professor and director of the Center for Research on Minority Health at the University of Texas M.D. Anderson Cancer Center. “Given the higher mortality rates for African American males in the United States, these results re-emphasize the need to determine all factors, including environmental contaminants, that are responsible for this continuing health disparity.”

“Given the importance of reproduction for the health of any species, the trends we observed in the United States and Japan merit concern,” added Davis. “In light of our findings, more detailed studies should be carried out that examine sex ratio in smaller groups with defined exposures as a potential indicator of environmental contamination.”

The study was supported by the Heinz Endowments, the University of Pittsburgh Cancer Institute, DSF Charitable Trust, the U.S. Centers for Disease Control and Prevention, and the W. Alton Jones Foundation.