The simultaneous effects of several variables on the secondary sex ratio have been examined using data from over 3.7 million births which occurred in Japan during 1975--6. A weak and negative association between sex ratio and birth order was observed but it was not significant in the statistical sense. A negative effect of paternal age--birth order interaction was obtained when maternal age was controlled. The quadratic model is much more powerful than the linear model in explaining the sex ratio variability.
Adolescent Adult