NCPA - National Center for Policy Analysis

Case of the Disappearing Boys

April 26, 2000

Slightly more boys than girls are born in the United States, but the percentage of baby boys appears to have gradually fallen during the last half of this century. The actual decline represents less than 1 percent of births. A similar trend is evident in other industrialized countries.

The theory that gets the most attention maintains that environmental pollution is toxic to some males in the womb. For instance, in a 1998 article in the Journal of the American Medical Association, Devra Davis of the World Resources Institute proposed that "reduced male proportion at birth be viewed as a sentinel health event that may be linked to environmental factors."

But scientists say babies' gender can be explained by benign factors, including the age of parents, family size, birth order, and so forth.

For instance, a report last month in Fertility and Sterility concluded that the decline in male births in the U.S. could be partly explained by the increasing age of parents:

  • Older parents, fathers especially, were more likely to have girls, and the association was particularly strong among white couples.
  • In fact, about 28 percent of the decline since 1964 among white births could be accounted for by changes in fathers' ages.
  • An additional 14 percent of the decline could be explained by taking into account the changes in mothers' ages.

Additionally, a National Cancer Institute study in the American Journal of Epidemiology of births in Denmark from 1960 to 1994 found that if every household had only one child, 51.2 percent of births each year would be male. If families got larger, that percentage would go up -- therefore, as family size shrinks in industrialized countries, the fraction of births that are male falls.

Source: Laura Beil, "Where the boys aren't," Dallas Morning News, April 24, 2000.


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