The advantages of analysing human variation using twins and twin half-sibs and cousins.

The "twin family" design is a new strategy for studying quantitative characters in man which overcomes most limitations of earlier designs and which is readily accessible to existing twin research units as it uses only adult twins (both identical and non-identical) and their spouses and offspring and juvenile twins (both identical and non-identical) and their parents. The design yields all information inherent in ordinary twin studies but also permits the simultaneous estimation of more components than any other design. Tests for most genetical and environmental components of variation and the assumptions of the design are provided. Particular advantages are the unambiguous separation of sex-linkage and maternal inheritance, the analysis of the mechanism of assortative mating and the specification of more realistic environmental models. Although several components are confounded the biases are not seriously misleading. However, it would be necessary to include adoption data to resolve the effects of cultural transmission which are otherwise confounded with the family environment. Nevertheless, this design provides a wealth of data on a diversity of relationships and promises to be a valuable tool for the analysis of individual differences in man.