What's happening to the baby boys?
Are they falling victim to something in the
environment unfavorable to the conception
of males - or the survival of male fetuses?
Is it something their moms or dads ate,
inhaled in the air, or took as medication?
Have sex practices changed in ways that discriminate
against the conception of male fetuses?
Experts on population in the United States, Canada, and
other industrialized countries are asking those questions
after discovering a long-term decline in the proportion of
male births. Some are calling this real-life medical
whodunit "the mystery of the missing boys."
For decades, the proportion of male-to-female births -
also called the "sex ratio" - has been etched in stone. It
remained at a steady 1.06 male births for every 1.0
female births. Then the ratio began to decline about 50
years ago. Between 1950 and 1994, for example, the
proportion of males decreased by 0.001 in the United
States, 0.002 in Canada, 0.003 in Denmark.
Yes, those are tiny declines. Yet demographers, scientists
who study populations, still regard them as very
important. Variations of fractions of percentage points
can be important in large populations.
In the United States, for instance, the 0.001 decline
translates into a loss of 38,000 males over the last 20
years. They are males who would have been born had the
sex ratio remained at its traditional level. Canada lost
about 8,600 male births during the same period.
Studies have established that the declines are
"statistically significant." That means the decreases
probably are due to real factors that somehow reduce the
number of male births, rather than chance.
There are plenty of hypotheses, but no solid explanation.
Dr. Devra L. Davis, for instance, argued that health
experts should consider the decline a "sentinel health
event," an early warning sign like the "canary in the coal
mine" of yet-undetected environmental pollution hazards.
Dr. Davis, a program director at the World Resources
Institute in Washington, co-authored a major study on the
situation.
She suggested, for instance, that some environmental
contaminant may block a father's ability to produce sperm
cells with a "Y" chromosome. That's the chromosome that
produces a male fetus. Another possibility: Some
environmental chemical may cause a higher number of
miscarriages among already-frail male embryos.
Frail male embryos? Yes, indeed. About 125 males are
conceived for every 100 females. Yet only 106 males are
born for every 100 females. Males really are the weaker
sex from the time of conception onward. They have a
shorter adult life expectancy and higher mortality rates
from many diseases.
Other studies, including those from the U.S. Centers for
Disease Control and Prevention (CDC), question whether
environmental pollution is the culprit. The proportion of
male births has declined in 16 industrialized countries.
CDC researchers think that nationwide changes in the
frequency or timing of sexual intercourse among married
couples may be involved. Those factors may have a
little-understood influence on the sex ratio.
Some studies suggest, for instance, that male conceptions
are more likely when fertilization occurs early in a
woman's menstrual cycle. That's the point when certain
hormone levels seem to favor a male conception. If
couples are having sex less often, fertilization is less
likely to occur during this brief male-birth-friendly time.
The fertilization factor, some experts believe, may
explain why the proportion of male births increases after
world wars, when males return home.
Michael Woods is The Blade's science editor.
http://www.newsweek.com/nw-srv/printed/us/na/a19230-2000apr30.htm