The La Trobe Twin Study: a genetic approach to the structure and development of cognition in twin children.

Twins are particularly useful for developmental behavior genetics, if adequate consideration is given to the possibilities of twin-singleton differences and of data from twins being unrepresentative of the entire population. As an alternative to existing analyses in the developmental behavior genetics of cognition, the covariance structure analysis method in biometrical genetics involving twins and other relatives has the potential for dissecting the determinants of the structure of cognitive abilities and how these change during childhood, as well as checking on the adequacy of twin data. Designed with these aims in mind, the La Trobe Twin Study is a mixed longitudinal analysis of 1,356 twins, their siblings, and cousins, who are followed between the ages of 3 and 15 years on a battery of physical and behavioral tests. The criteria for choosing behavioral tests in such a project are discussed, along with the comprehensive series of questionnaires necessary to examine the children's development in the context of the stresses a multiple birth may impose on a family. In discussion of methods of analysis, 2 points are emphasized: (1) Twin-singleton differences are specific to particular abilities rather than being general, and (2) the incidence of family changes and stresses in such that these should be considered among the environmental effects on development along with the more usual indices of family environment.