The Inheritance of Human Birth Weight*

Summary Japanese data on half‐sibs, twins, full sibs and consanguineous matings are consistent with the hypothesis that the resemblance in birth weight of sibs is largely attributable to the maternal constitution or environment, not to genetic similarity of sibs. The following intraclass correlations were obtained: paternal half‐sibs, 0·102; maternal half‐sibs adjacent in birth order. 0·581; like‐sexed twins, 0·557; unlike‐sexed twins, 0·655; full sibs adjacent in birth order. 0·523; full sibs with one sib intervening, 0·425; full sibs with two sibs intervening. 0·363; full sibs adjacent in birth order, whose parents are first cousins, 0·481. No significant effect of inbreeding was observed on the variance of birth weight or the correlation between sibs, but inbreeding of the foetus did appear to affect mean birth weight, 10% inbreeding being estimated to decrease birth weight by about 4·3 decagrams. Other data on the genetics of mammalian body weight are reviewed.