newer line of research, but data from larger-scale twin studies, for example, have indicated that the covariance between RT data and ADHD scores in the general population is independent of the covariation between ADHD scores and lowered IQ.62 To date, a multivariate genetic approach with twin data has not been employed in the development of endophenotypes for ADHD, and replication remains a key outstanding issue. Nevertheless, the potential value of twin studies to endophenotype and phenotype definitions seems clear given these interesting and promising initial results.
