Declines in Population Sex Ratios at Birth

KEYWORDS:
• abnormalities
• birth defects
• environmental exposure
• population
• sex ratio

To the Editor: Dr Davis and colleagues¹ cite evidence that the sex ratio (proportion of males at birth) has been diminishing during the last few decades in the United States, Canada, Sweden, Germany, Norway, Finland, Denmark, the Netherlands, and some countries in South America. The authors also cite evidence that declines in sex ratio (mainly male) are associated with exposure to a number of deleterious chemicals, including dioxin, the nematocide dibromochloropropane and other pesticides, borates, alcohol, lead, solvents, and waste anesthetic gases. They raise the question of whether the decline was attributable to exposure to these chemicals. To answer that question, we need to know what the population sex ratios would have been in the absence of those chemical agents.

Gini² showed that population sex ratios typically varied up and down slowly across time well before the introduction of those agents. These variations were statistically significant but remain …