A genetic analysis of alcohol consumption in 3810 pairs of adult twins is reported. When no correction was made for age, individual environmental variance, including non-repeatable errors of reporting, accounted for approximately 44% of variation in both sexes. In females, there was no evidence of shared environmental effects and 56% of the variance was genetic in origin. In males, only 36% of the variance was genetic and common environmental effects accounted for the remaining 20% of individual differences.

For females, the results for younger (30 years and under) and older (over 30) twins were similar. For males, however, the effect of age was striking. In younger male twins over 60% of the variance was genetic in origin, with the remaining variance due to environmental influences unique to the indiviudal. In older twins genetic differences do not appear to be important, with approximately 50% of the total variance due to individual environmental differences and the remaining 50% due to the effect of the common family environment. Our results suggest that both age and sex need to be considered when analysing the causes of variation in alcohol consumption.

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.