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


  Mackenzie, CA, A Lockridge and M Keith. 2005. Declining Sex Ratio in a First Nation Community. Environmental Health Perspectives 113: 1295-1298.

Mackenzie et al. report in this study a striking multi-year decline in the sex ratio of a First Nations community, the Aamjiwnaang, living immediately adjacent to the Sarnia-Lambton Chemical Valley--"one of Canada's largest concentrations of industry." Data they present reveal that the sex ratio of this community was stable and normal until 1993, at which point it began to drop. For the 5-year period beginning in 1999, fewer than 35% of live births were boys compared to the expected of just over 50%. Statistical analysis shows these changes to be highly significant.

This stage of their research was not intended to test for possible causes of the decline, but rather simply to document trends and lay the groundwork for future studies.

Background on sex ratio

Around the world, slightly more boys are born than girls. This pattern is remarkably universal, with 51 to 54 boys born for each 50 girls born. Demographers use these data to calculate a population sex ratio, calculated as the number of male births divided by the total number of births.

Some demographers have reported small but significant changes in sex ratio in some countries, for example, in Canada and the US. Several studies have reported striking local changes in sex ratio associated with chemical exposures, for example in Russian pesticide workers, babies born to men who were exposed to dioxin in the 1976 Seveso (Italy) accident, and babies born to men in Taiwan who were exposed to PCBs while less than 20-years old.


What did they do? Mackenzie et al. worked with residents in the community to design and carry out surveys of the numbers of boys and girls born in the community for the years 1984-2003. Approximately 850 band members live in the Aamjiwnaang reserve land. Vital statistics for the community are reported monthly to the Canadian Department of Indian and Northern Affairs.

They calculated the sex ratio for each year during the study period, and then compared this to the Canadian average of 0.512. They also investigated the sex ratio in another community of Chippewa, who live away from Sarnia-Lambton Chemical Valley. It turns out that the sex ratio of this community does not differ from the Canadian average.

What did they find?

Prior to 1993, the sex ratio of the community was stable and not different from the Canadian average (red line, graph to right). But for the remainder of the decade and on to the most current data (2003), the sex ratio of this First Nations community plummeted.


adapted from Mackenzie et al. 2005

Mackenzie et al. performed several statistical analyses of these data.

  • Prior to 1992, there was no trend in sex ratio (p=0.990).
  • Examining data gathered since 1993, they found a sharp decline in sex ratio. The most pronounced decline was during the most recent five years (1999 to 2003; p=0.006).
  • The proportion of male live births for the most recent five year interval (1999 to 2003) was 0.348, much lower than the expected sex ratio of 0.512.

More on sex ratio
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What does it mean?

Mackenzie et al. have documented a large decline in the sex ratio of a First Nation community living near one of Canada's largest chemical industry complexes. Their data do not establish a cause for this decline. The high exposure levels this community is likely to be experiencing may be involved, and this would be consistent with some but not all prior studies.

Mackenzie et al. review relevant studies in their paper, and list a series of associations that have been reported in prior research:

  • Associated with decreases in sex ratio (fewer boys): Dioxin, PCBs, DBCP, HCB, methylmercury, air pollution from incinerators, maternal exposure to non-ionizing radiation, paternal exposure to high voltages, paternal occupation, maternal exposure to a pharmaceutical used in infertility treatment (clomiphene citrate) and parental smoking.
  • Associated with increases in sex ratio (fewer girls): PCB exposure, petrochemical air pollution, natural gas exposure, air pollution from a local steel foundry, and paternal exposure to ionizing radiation.
  • No associations: dioxin, PCBs and PCDFs, HCB, living near a petroleum refinery, general air pollution and background ionizing radiation.


It is entirely plausible that different compounds can act differently, through the same or different mechanisms. Hence the diversity of pattern noted above is not surprising, although some of the results would appear to be in conflict with one another. Resolving that will require a closer look at the specific congeners of chemicals involved and also, in most cases, much better estimates of exposure. The less accurate the exposure measurement, the more likely it is that no effect will be seen, even if more accurate measurements would reveal one. Parental occupational classifications used to estimate exposure are especially weak.





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