in human sex ratio
the sex ratio (numbers of boys born divided by the numbers of
girls born) is slightly greater than one. In fact, world-wide
about 106 boys are born for every 100 girls. This number is also
reported as the male proportion of total births, or 106/206 =
0.514 = 51.4%.
researchers, however, have reported apparent recent declines in
the proportion of male births, in the US, Canada, Denmark and
the Netherlands. These declines have been very small but statistically
significant. Fewer boys are being born than would be expected
on the basis of the recent historical worldwide average.
addition to these general declines, several specific cases have
been reported involving sharp alterations in the sex ratio of
people because of industrial accident, occupational exposure,
or because of exposure to air pollution from incinerators. For
example, in Russia pesticide workers
exposed to elevated dioxin and dioxin-like compounds are less
likely to father boys. A similar pattern is observed in men exposed
to a mixture of PCBs in Taiwan.
Both these studies report no effect on the sex ratio of infants
born to exposed women. The Taiwanese study also suggests that
exposure before the age of 20 is necessary for there to be an
effect on sex ratio.
Most recently, public health scientists in Canada have reported a sharp drop in the sex ratio of a First Nation community (Chippewa) near Sarnia, Ontario, a heavily industrialized region with the province. Over fewer than 10 years, the ratio dropped from the norm, around 50:50, to roughly one-third boys, two-thirds girls. The cause of this decline has not been identified, but the pattern is very strong.
Most of the discussions of the country-level changes noted above have treated it as small shifts in the individual probability of one sex vs the other over the population as a whole. Examples like Sarnia, however, propose another pattern: that a small growing number of 'hot spots' where sex ratio changes are large pull down the population average.
If the former, then while it may be an interesting indicator of chemical contamination (or some other factor) it is difficult to see immediate public health consequences. But if the later, it raises acute questions about the consequences within those hot spots for public health.
P, PM Gerthoux, E Ferrari, DG Petterson, SM Kieszak, P Brambilla,
N Vincoli, S Signorini, P Tramacere, V Carreri, EJ Sampson, WE Turner
and LL Needham. 2000. Paternal concentrations of dioxin and sex
ratio of offspring. The Lancet 355:1858-1863.
Mocarelli et al. revisit their data (see
below for earlier paper) on the sex of children born to parents
exposed to dioxin in the 1976 dioxin accident in Seveso, Italy,
and carry the analysis farther. They find that the father's exposure
level is the best predictor of sex of offspring. Higher dioxin
exposure of the father decreased the likelihood of having sons compared
the father's nor the mother's age at conception was related to sex
of children born controlling for dioxin exposure. But fathers exposed
at a young age (younger than 19) were significantly more likely
to have daughters than sons if their exposure to dioxin was relatively
experimental data and our observation of the lower sex ratio among
the offspring of men who were exposed to dioxin during adulthood
or before and during puberty support the hypothesis that dioxin
permanently affects the human epididymus from the time of exposure."
H. 1998. Trends in sex-ratio, testicular cancer and male reproductive
hazards: Are they connected? APMIS 106:232-239.
presents data from a Danish case-control study showing a strong
association between testicular cancer, low fertility and an excess
of females compared to males among offspring. He hypothesizes that
common causal factors may underlie these associations, specifically
that agents acting prenatally to disrupt normal development may
be involved. More...
DL, MG Gottlieb and JR Stampnitzky. 1998. Reduced ratio of male
to female births in several industrial countries: A sentinel health
indicator? Journal of the American Medical Association 279:1018-1023.
et al. examine birth records on the proportion of males born
in Denmark, the Netherlands, Canada and the United States. They
find that the sex ratio has declined in each of these countries.
As with Møller's finding, above (and below), the percentage
change is small but statistically significant. Davis et al. observe
that the change observed in the US over a twenty year period amount
to a cumulative decrease of 1 male birth per 1000 live births They
go on to calculate that this corresponds, given actual birthrates,
to an absence of 38,000 male births.
et al. also summarize medical and scientific literature on
known and hypothesized causes of changes in sex ratio They cite
a series of studies of contamination that have resulted in declines
in the proportion of males born. The examples include dibromochloropropane,
exposures of fathers working in the aluminum industry, organochlorine
pesticides and waste anesthetic gases.
conclude by proposing that "reduced male proportion at birth
be viewed as a sentinel health event that may be linked to environmental
H. 1996. Change in male:female ratio among newborn infants in
Denmark. The Lancet 348:828-829.
reports that the proportion of boys among newborn infants decreased
in Denmark from the 1960s to the present, after having risen from
1850 to 1950. According to Møller, the initial increase is
"mainly a consequence of decreasing stillbirth rates and the
decreasing male excess among stillbirths." He cautiously proposes;
"It is possible that environmental or other agents with toxic
properties on the male reproductive system could lead to low male:female
ratios" and he cites the nematocide dibromochloropropane (DBCP)
as one example among exposed men.
P, P Brambilla, PM Gerthous, DG Patterson, Jr. and LI Needham. 1996.
Change in sex ratio with exposure to dioxin. The Lancet
Italy, stunning evidence of reproductive effects in humans has emerged
from the ongoing investigation of the human health consequences
of the explosion at a chemical factory in Seveso in 1976. This latest
report raises more questions than it answers. The dioxin released
in this accident contaminated thousand of people in the surrounding
city and left some with the highest dioxin levels ever measured
in humans. As reported in Our Stolen Future (Chapter 7), the follow-up
studies had initially focused on cancer rates in the accident victims
until the endocrine disruption theory recently raised new questions
about possible effects on their children.
examining the children born to to parents with high dioxin exposure
from April 1977 (one year following the accident) to December 1984,
the research team discovered a highly skewed sex ratio. Instead
of the typical ratio in humans of 106 males to 100 females, these
parents produced only 26 boys to 48 girls. The scientists have no
explanation for such a shift, but they note evidence that normal
sex ratios are maintained through the hormone concentrations in