Studies of twins and other family relatives continue to offer a great deal to ADHD research. Univariate analyses provide a “target” of genetic variation to be accounted for by molecular genetic studies, but this is a small fraction of the potential value of multivariate analyses of twin and family data. The latter offer a clear route to the identification of a small number of latent, substantially or entirely genetic factors. Estimation of individual factor scores of these latent traits—which may underlie variation in objective and subjective measurements of ADHD phenotypes—is a straightforward statistical procedure. These scores in turn seem likely to prove valuable in the identification of single nucleotide polymorphisms that generate individual differences in liability to ADHD. A further advantage is that twin studies permit some resolution between “true” endophenotypes, which are intermediate between genotype and ADHD phenotype, and simple correlates of ADHD, which share some of their causal factors.
