Male births declining in US and Japan

Mother Nature has always ensured that male births outnumber female ones, but the gap has been gradually narrowing over the past three decades in the U.S. and Japan, according to a new study.

Researchers suspect the decline in male births can be explained, at least in part, by paternal exposure to environmental toxins, such as certain pesticides, heavy metals, solvents or dioxins -- chemical byproducts produced during incineration or the manufacture of other chemicals.

Traditionally, it's been expected that for every 100 girls born, there will be about 105 boys. But since 1970, the U.S. and Japan have experienced a downward shift in this male-to-female birth ratio, researchers report in the online edition of the journal Environmental Health Perspectives.

In the U.S., the proportion of boys dropped from 105.5 per 100 girls in 1970 to 104.6 in 2001; in Japan, the male-to-female ratio dropped from 106.3 boys for every 100 girls to just fewer than 105 per 100.

The changes may seem small, but the study authors suspect they are one manifestation of the effects of environmental pollutants on the male reproductive system.

The decline in male births has occurred "at the same time that we've been seeing other signs that male reproductive health is in danger," said lead study author Dr. Devra Lee Davis, a professor of epidemiology at the University of Pittsburgh Graduate School of Public Health.

These other signs, she said, include lower testosterone levels and sperm counts, as well as increases in testicular cancer, a disease that most often affects young men.

Environmental toxins may be a common denominator here, according to Davis and her colleagues. Such exposures may specifically lower rates of male, rather than female, births for a few reasons. They may, for example, affect the viability of sperm that bear the Y chromosome, which determines male sex -- or the viability of male fetuses.

Davis's team found that while fetal deaths have declined overall in recent decades, the proportion of male deaths is growing. In Japan, in particular, male fetuses accounted for about two thirds of all fetal deaths in the 1990s.

Over the years, there have been a number of reports showing that heavy exposure to certain pollutants may affect a man's likelihood of fathering a son.