As we reported late in 1997 (REHW #576), the male proportion of live
births has been declining in the U.S. and Canada for at least 20 years.
Normally there are 106 males born for every 100 females, for a male
proportion of 106/206 = 0.5145. This proportion is often called the
"live birth sex ratio" or simply the "sex ratio." Several studies --
some very recent[1,2] and others dating back to the late 1980s and
early 1990s --reveal similar 20-year declines in the male proportion of
live births in England and Wales, Denmark, Sweden, Finland, the
Netherlands, Germany, Chile, Argentina, Brazil, Bolivia, Peru,
Paraguay, Ecuador, Venezuela, Colombia, and Costa Rica.
The declines in the male proportion of births are not large, and by
themselves they have no practical importance. Often the shift has been
from 0.515 to 0.513 or 0.512. Nevertheless, even small shifts in the
sex ratio can add up. In Canada during the last 20 years, approximately
8600 males have not been born and in their place we have females; in
the U.S. during the same period, 38,000 baby boys were replaced by baby
girls. (From an environmental perspective this is arguably beneficial
because environmentally destructive organizations tend to be dominated
by men, while the effective wing of the environmental movement is
largely led by women.)
Nevertheless, there is a very serious side to these small shifts in sex
ratio. It is extremely unlikely that similar trends in so many
different countries are due to chance. Therefore, the altered sex ratio
raises a grave public health question: what is causing consistent
biological changes in the human populations of so many industrialized
and industrializing countries? Does this pattern signal some general
change in exposures to environmental toxicants? Now two teams of
researchers --in the U.S. and in Denmark --are asking whether the
pattern of declining male proportion of births is linked to a similar
pattern of increasing birth defects of the penis and testicles,
increasing testicular cancer, and declining quality and quantity of
Both groups of scientists are hypothesizing that all of these patterns
are linked to exposures to hormone-disrupting chemicals including
dioxin, pesticides, lead, solvents and smoke stack emissions from
smelters, steel foundries and incinerators. And both groups are
hypothesizing that the relevant exposures are most likely taking place
before birth, in the mother's womb.
Devra Lee Davis of the World Resources Institute (Washington, D.C),
writing in the JOURNAL OF THE AMERICAN MEDICAL ASSOCIATION,
acknowledges that any change in a complex biological process, such as
sex determination, probably has several causes. She then goes on to
argue, offering evidence from many different areas of research, that
the male proportion of live births can be affected by environmental
** At the moment of conception, all embryos are destined to be female
unless something changes them into males. We all start out female by
default. For the first six to 9 weeks of life, we all have unisex
gonads. Between the 6th and 9th week, the gonads of those with a Y
chromosome specialize into testicles and begin producing hormones that
continue the process of creating a male. If anything interferes at this
stage, a female may result. Thus sex is determined by tiny amounts of
hormones circulating in the blood of the embryo. This situation
provides opportunities for chemicals entering the mother's body
("xenobiotics") to disrupt normal processes. And there is some evidence
that chemicals entering the mother's body do just that:
** In Seveso, Italy, in 1976, a group of people were heavily exposed to
dioxin after an explosion at a pesticide factory. Dioxin is a potent
poison, an unwanted byproduct of metal smelters, the manufacture of
paper and pesticides, and incinerators. Between April 1977 and December
1984, 74 children were born to parents in the zone of greatest dioxin
exposure. Of these, 48 were female and 26 male, for a male proportion
of 0.351. After 1985, the male proportion of live births began to
return to normal.
** Among a group of workers applying the pesticide dibromochloropropane
(DBCP), several men became sterile. Importantly, those able to have
children produced 3 times as many daughters as expected.
** A study of pesticide-exposed workers in the Netherlands revealed a
male proportion of 0.248, less than half the normal proportion of male
** Five studies of heavily polluted residential areas in Scotland
revealed significantly diminished sex ratios. The pollutants were
emitted by metal smelters, steel foundries, and incinerators.
** In rural Minnesota, increased rates of birth defects are reported to
occur among the male children of workers who apply pesticides,
suggesting that the male fetus may be especially vulnerable to hormone-
disrupting substances. In an area with high usage of chlorophenoxy
herbicides and/or fungicides, the male proportion among children born
with defects to workers who apply pesticides was 0.735, compared to a
male proportion of 0.607 for births with defects among the general
Davis considers many other factors that can reduce the male proportion
of live births --age differences between the parents, older age of
father, mother under stress, multiple sclerosis, less-frequent
intercourse, and test-tube fertilizations. None of these factors
appears likely to have operated consistently in so many countries for
20 years or more. Therefore, Davis proposes that altered male
proportion of live births should be considered a "sentinel health event
that may be linked to environmental factors." A sentinel health event
is defined as "an unusual pattern of health in a population that
signals changes in avoidable factors. Thus," Davis writes, "changes in
either a relatively common health occurrence, such as childhood asthma,
or a relatively rare disease, such as pulmonary hypertension, can
reflect changes in avoidable exposures."
The sex ratio is not static. In many countries, the ratio increased
between 1900 and 1950 as better prenatal care reduced the number of
stillbirths, which tend to affect males disproportionately. Thus,
modern medicine has been able to avoid stillbirths, thereby keeping
more baby boys alive. But some time between 1950 and 1970 the male
proportion of live births began to decline. Henrik Moller of the Danish
National Research Foundation offers arguments similar to those of
Davis, but based on somewhat different evidence. Moller restricts his
discussion to men in Denmark, Finland, Norway, and Sweden. He argues
that the declining male proportion of live births in these countries is
closely linked to testicular cancer, and to declining sperm quality and
quantity. He suggests that all of these effects may be caused by
prenatal exposures to chemicals that act like dioxin and the pesticide
DBCP, mentioned above.
Between 1960 and 1990, the incidence of testicular cancer doubled or
more-than-doubled in each of the four Nordic countries. During the same
period, testicular cancer also increased in England and Wales,
Scotland, Australia, New Zealand, Slovenia [former Yugoslavia], Poland,
Spain, Colombia, Japan, India, and the U.S. Some of these increases
may be due to better diagnosis, but the increases were consistent in
all populations, were of similar size, and were reported by long-
established cancer registries. Testicular cancer is a well-defined
entity, and it inevitably becomes apparent, so it is unlikely to be
Moller and others argue that, because testicular cancer tends to occur
young in life, whatever causes it must become effective early, perhaps
even before birth. The causes of testicular cancer are poorly
understood. One recent study has found that occupational exposure to
polyvinyl chloride (PVC) increases the likelihood of getting testicular
In the four Nordic countries that Moller discusses, during the period
that testicular cancer has been increasing, the quality of sperm has
declined. Sperm quality and quantity have also declined (on
average) in the U.S. and in other European countries. Moller argues
that all these trends are related.
Moller conducted a case-control study of 514 Danish men with testicular
cancer, comparing them to a control group of 720 men without cancer.
In both groups Moller interviewed the men and their mothers. Among the
cancer cases, he recorded the sex of their children born up until 2
years before their cancers were diagnosed. He found a male proportion
of 0.470. Among children born to the 720 controls, Moller found a male
proportion of 0.520. Thus there was a significant reduction in the male
proportion of live births among men who would later develop testicular
cancer. Moller says his study does not close the book on the subject --
there are two other studies on record which found different results.
In recent years the world's scientific community has been expressing
concerns about male reproductive health --increasing birth defects of
the penis and testicles, declining sperm quantity and quality, and
increasing testicular cancer --and asking whether these phenomena all
have common origins in some sort of chemical exposures. (See REHW #438,
#514.) This new information about altered sex ratios in more than a
dozen countries simply makes the case more compelling and more urgent.
--Peter Montague (National Writers Union, UAW Local 1981/AFL-CIO)
 Devra Lee Davis and others, "Reduced Ratio of Male to Female Births
in Several Industrial Countries," JOURNAL OF THE AMERICAN MEDICAL
ASSOCIATION Vol. 279, No. 13 (April 1, 1998), pgs. 1018-1023.
 Henrik Moller, "Trends in sex-ratio, testicular cancer and male
reproductive hazards: Are they connected?" APMIS [ACTA PATHOLOGICA,
MICROBIOLOGICA ET IMMUNOLOGICA SCANDINAVICA] Vol. 106 (1998) pgs. 232-
 H.O. Dickinson and L. Parker, "Why is the sex ratio falling in
England and Wales? " JOURNAL OF EPIDEMIOLOGY AND COMMUNITY HEALTH Vol.
50 (1996), pgs. 227-230.
 M.F. Feitosa and H. Krieger, "Demography of the Human Sex Ratio on
Some Latin American Countries," HUMAN BIOLOGY Vol. 64, No. 4 (August
1992), pgs. 523-530.
 F.L.R. Williams and others, "Low Sex Ratios of Births in Areas at
Risk From Air Pollution from Incinerators, as Shown by Geographical
Analysis and 3-Dimensional Mapping," INTERNATIONAL JOURNAL OF
EPIDEMIOLOGY Vol. 21, No. 2 (1992), pgs. 311-319.
 G. Potashnik and others, "Dibromochloropropane-induced Reduction of
the Sex-ratio in Man." ANDROLOGIA Vol. 16, No. 3 (1984), pgs.213-218.
 David Forman and Henrik Moller, "Testicular Cancer." CANCER SURVEYS
Vol. 19/20 pgs. 323-341.
 Anders Ekbom and Olof Akre, "Increasing incidence of testicular
cancer --birth cohort effects," APMIS [ACTA PATHOLOGICA MICROBIOLOGICA
ET IMMUNOLOGICA SCANDINAVICA] Vol. 106 (1998). pgs. 225-231. And see H.
Moller, "Clues to the Aetiology of Testicular Germ Cell Tumours from
Descriptive Epidemiology," EUROPEAN UROLOGY Vol. 23 (1993), pgs. 8-13.
 Lennart Hardell and others, "Occupational exposure to polyvinyl
chloride as a risk factor for testicular cancer evaluated in a case-
control study," INTERNATIONAL JOURNAL OF CANCER Vol. 73 (1997), pgs.
 Erik Bendvold, "Semen Quality in Norwegian Men over a 20-Year
Period," INTERNATIONAL JOURNAL OF FERTILITY Vol. 34, No. 6, (1989),
pgs. 401-404. And: Erik Bostofte and others, "Has the Fertility of
Danish Men Declined Through the Years in Terms of Semen Quality? A
Comparison of Semen Qualities between 1952 and 1972," INTERNATIONAL
JOURNAL OF FERTILITY Vol. 28, No. 2, (1983), pgs. 91-95.
 Shanna H. Swan and others, "Have sperm densities declined? A
reanalysis of the global trend data," ENVIRONMENTAL HEALTH PERSPECTIVES
Vol. 105 (1997), pgs. 1228-1232.
Descriptor terms: sex ratio; male reproductive health; testicular
cancer; cancer; carcinogens; pvc; semen; birth defects; cryptorchidism;
undescended testicles; devra davis; england; denmark; sweden; finland;
netherlands; germany; chile; argentina; brazil; bolivia; peru;
paraguay; ecuador; venezuela; colombia; cost rica; canada; u.s.;
hormone-disrupters; wri; jama; dibromochloropropane; dbcp; pesticides;
kenrik moller; norway; wales; scotland; incineration; australia; new
zealand; slovenia; poland; spain; japan; india;