title: The secondary sex ratio, paternal age, maternal age and birth order in Japan.
abstract: 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.
mesh_terms: D000293:Adolescent; D000328:Adult; D001722:Birth Order; D005260:Female; D006801:Humans; D007564:Japan; D008297:Male; D008423:Maternal Age; D008875:Middle Aged; D008954:Models, Biological; D010331:Paternal Age; D012044:Regression Analysis; D012744:Sex Ratio