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
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
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
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