In an attempt to uncover the causes of variation in birth weight for 13,970 offspring of MZ and DZ twins, several models were tested. Mean squares from nested analysis of variance were analysed with respect to fetal and maternal effects on variation in birth weight. The major part of the total variation in birth weight was found to be due to effects of genes. The contribution of fetal genes was larger than the contribution of maternal genes. About 11% of the variation could be attributed to effects of interactions between fetal and maternal genes. However, in this data set, the interaction variance could not be distinguished from variance due to fetal dominance or to effects of common environment of sibs.
