PIP: The author studied demographic data from the U.S., France, Chile, England, and Wales, considering birth rate, mortality, sex ratio and male proportions in quinquennial periods from 1960 to 1977. Moreover, the author has assumed that the difference in sex ratio between live births and fetal deaths is due to X-linked mutations, and has estimated the number of males eliminated by differential mortality. Results showed that, in correlation with birth rate decrease, there is a trend of relaxation of intrauterine natural selection against males, which trend accounts for the change of sex ratio at different ages in the perinatal period. This sequence of events is fully adapted to the sex linked genetic theory. The author concludes that when birth rate declines there is a fall in the sex ratio of stillbirths, an increase of sex ratio in live births and infant mortality, and consequently a relaxation of natural selection against males.