In addition to COT, extended twin designs that incorporate information on spouses of twins (i.e. to test for the assumption of random mating, or whether spousal correlations for behaviors like smoking and drinking is attributable to shared genes and environments) as well as parents and non-twin siblings have been employed 29. Using, primarily, the classical twin design and its multivariate extensions, a host of studies have examined the sources of individual differences in alcohol, tobacco and illicit drug involvement.
