The secondary sex ratio in the United States 1969-71: association with race, parental ages, birth order, paternal education and legitimacy.

The simultaneous effects of several variables on the secondary sex ratio have been examined using data from over 5,000,000 births which occurred in the United States during 1969-71. The previously described negative association of birth order and sex ratio has been confirmed. For legitimate and illegitimate births combined, maternal age and paternal age are unimportant factors once account is made of birth order. This pattern obtains for both blacks and whites. The sex ratio for legitimate and illegitimate births is equivalent but the fact of legitimacy or illegitimacy may affect the association of the ratio with birth order and the parental ages. Paternal education is not significantly related to the ratio, but the highest probability of a male birth is found among fathers with intermediate levels of attainment. Even though the association between sex ratio and order of birth is highly significant in the statistical sense, the proportion of male births changes less than 2% over the extreme values of birth order. Furthermore, it is emphasized that the association accounts for only a very minor (less than 10%) proportion of the total variation in the ratio.