Birth order, parental ages, and sex of off-spring.

Abstract

Analysis of 1 year of data (1955) including about 4 million births i n the U.S. attempts to assess the relative contributions of age of fathe r age of mother and birth order to the sex of offspring. An equation was determined to fit a regression function of sex ratio on all 3 variables simultaneously. The magnitude of the effect under study was very small about 1%. Variation in secondary sex ratio is independent of the mothers age but is dependent on both birth order and paternal age. Birth order and paternal age may be themselves correlated with other factors more closely related to the true causes of the variations in sex ratio. It is suggested that the relationships are nonlinear and cannot be represented adequately without inclusion of a component that reflects an interaction between birth order and age of the father.