Our Stolen Futurea book by Theo Colborn, Dianne Dumanoski, and John Peterson Myers



Changes in human sex ratio

Usually, 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%.

Several 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.

In 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.

Earlier studies include:

Mocarelli, 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 to daughters.

Neither 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 high.

"These 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."


Møller, H. 1998. Trends in sex-ratio, testicular cancer and male reproductive hazards: Are they connected? APMIS 106:232-239.

Møller 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...


Davis, 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.

Davis 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.

Davis 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.

They conclude by proposing that "reduced male proportion at birth be viewed as a sentinel health event that may be linked to environmental factors."

Møller, H. 1996. Change in male:female ratio among newborn infants in Denmark. The Lancet 348:828-829.

Møller 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.


Mocarelli, P, P Brambilla, PM Gerthous, DG Patterson, Jr. and LI Needham. 1996. Change in sex ratio with exposure to dioxin. The Lancet 348:409.

In 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.

In 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 the parents.






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