Fact Sheets

Sex Ratios

Issue: The number of human males, relative to females, born in North America and elsewhere in the world has been declining for several decades. The cause of this change is unknown however environmental chemical exposure is suspected to play a role.

Background: The sex ratio is calculated by dividing the number of live male births by the total number of births for a given period of time. Although there is poor understanding of factors that alter the sex ratio, there is clear evidence that external influences are associated with such a change. These factors can be grouped into medical, occupational and environmental causes.

The reported medical factors shown or suggested to reduce the male proportion of offspring include older age fathers and mothers, in vitro fertilization, ovulation induction, non-Hodgkin's lymphoma, hepatitis and multiple sclerosis. Also, men who develop testicular cancer tend to father more female children than are characteristic of men in the general population.

A study of offspring born from 1978 to 1990 in the Netherlands revealed an increase in daughters when men had workplace exposure to pesticides. Other occupational exposures reported to be associated with a change in the sex ratio have included working in the aluminum industry as "carbon setters", "anode setters" or "carbon changers" and exposure to waste anesthetic gases.

Changes in the sex ratio have been reported following the accidental release of dioxin into the environment in Seveso, Italy. Although these data suggest an association between exposure to an environmental contaminant and altered birth sex ratio this report must be interpreted with caution due to the relatively small sample size involved. Moreover, this finding has not been supported by observations of altered sex ratios of children from other dioxin exposed populations.

Trends: Sex ratios in Canada, United States, Denmark and the Netherlands have been shown to decrease over a period of between 20 to 30 years. Similar changes have been reported for other countries although these data must be regarded with caution due to sample size limitations.

Consistency of the data: Although a decreasing trend in sex ratio has been reported in the Netherlands in one study, a trend towards an increase in the sex ratio has been reported for Italy, Greece and the Netherlands. Regression analysis of temporal trends in sex ratio of live births between 1969 and 1995 in the United States revealed a significant decline among whites for the 27 years under study (OR=0.9935; 95% CI 0.9919 - 0.9952). In contrast the sex ratio among blacks during the same time period revealed a significant increase in the sex ratio (OR 1.0208, 95% CI 1.0162 - 1.0254). There is also a suggestion in the literature that changes in sex ratio may precede environmental contamination with industrial chemicals.

Experimental evidence: In animal studies the sex ratio of animals treated with various test compounds has been shown to be altered. However, apart from documenting a change in the sex ratio of the offspring no insight has been provided as to a possible mechanism for the observed change.

Biological plausibility: Very little information is present in the literature that addresses the factors affecting the sex ratio. Two separate hypotheses provide limited insight into the mechanism(s) affecting sex ratio in animals. It has been suggested that the developmental rate of the conceptus differs on the basis of genetic sex. It is assumed that shorter intervals between cell divisions increases a cells vulnerability to toxic insult and thus the more rapid growth of the male fetus increases susceptibility to lethal changes that will ultimately affect the sex ratio. Secondly, in mice, the position of a female fetus during pregnancy (between two males vs. between two females) has been shown to affect the sex ratio of her offspring. These data suggest that hormonal environment during gestation is a factor that subsequently affects sex ratio of the offspring when they begin having litters. However, the mechanism(s) underlying these changes have not been explored and both hypotheses have yet to be experimentally evaluated.

Current assessment of the weight of evidence: While there is evidence for a decrease in the number of males being born in some regions, the literature is equivocal with some reports demonstrating no change and others finding divergent effects depending on race. Furthermore, lacking a credible biological mechanism to explain the observed effects the overall conclusion is that the evidence linking exposure to environmental contaminants and changing sex ratio at this time is very weak.

Key Papers on this issue:

1. Davis, D.L., Gottlieb,M.B., Stampnitzky,J.R. (1998) Reduced ratio of male to female births in several industrial countries: A sentinel health indicator? JAMA 279:1018-1023. Get Summary.
   
   
2. Henrik Møller (1998) Trends in sex-ratio, testicular cancer and male reproductive hazards: Are they connected?APMIS 106:232-239. Get Summary