Sex ratio and selection by early mortality in humans: fifty-year analysis in different ethnic groups.

We studied the sex ratio (M/F) in representative populations of the main human ethnic groups, namely, US whites, US blacks, and Japanese. The data cover a period of over 50 years. For intra-ethnic comparison, we included analogous data on Italians. The populations studied show heterogeneous patterns: the US white and Italian populations are the most similar, with no drastic variations in live-birth sex ratio throughout the period. Comparison of sex ratio data for live-borns and 1-year-old infants yields a similar pattern in all groups; the differences between the two sex ratio values are constantly reducing. It can be hypothesized that in the near future the sex ratio observed at birth will be maintained up to reproductive age. If this value is stabilized by natural selection, as is likely, new relationships between the sex ratio and selection are expected to evolve in the populations of developed countries.