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Last Edited: 10/3/2005

Sex Ratio Among California Births, 1960-1996
01/1999 - 12/2005


The male sex ratio at birth (or the proportion of births in a population that is male) has been suggested as a sentinel environmental health indicator. This proportion is usually around 51%, but in offspring of persons with high chemical exposures, it can be dramatically decreased. Several recent publications from the U.S., Canada, and Europe have noted small but apparent declines in the male birth proportion among the general population. We used California births from 1960 through 1996 (over 15 million births) for which demographic data were obtained from the birth certificates.

Examining the raw data, the male birth proportion does indeed seem to be declining. However during this period, there have also been shifts in the demographics of California births, including age and ethnicity of the parents, which are known to influence the sex ratio, and so need to be controlled. Various multivariable regressions (linear, quadratic, and spline), adjusting for birth order and age and race of the parents, indicate that the decline seems mainly confined to Whites. But even this may be an artifact: separating the later births into Hispanic and Non-Hispanic Whites indicates that the increasing proportion of Hispanic births in California since the 1980s may explain the apparent trend.


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