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

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Abstract
The simultaneous effects of several variables on the secondary sex ratio were examined using data from over 5 million births which occurred in the USA during 1969-71. The previously described negative association of birth order and sex ratio was confirmed. For legitimate and illegitimate births combined, maternal age and paternal age are unimportant factors, once account is made of birth order. This pattern pertains to 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 birth order is highly significant in the statistical sense, the proportion of male births changes less than 2% over the extreme values of birth order. It is emphasized that the association accounts for only a very minor (< 10%) proportion of the total variation in the ratio.