Quantitative genetic analyses were used to estimate the relative contributions of genetic and environmental factors to use of cigarettes and use of cannabis, and the overlap between them. Classical twin models were used to estimate the role of additive genetic factors (A), shared environmental factors (C; environmental influences that make twins similar to each other), and non-shared environmental factors (E; environmental influences not shared by twins, as well as error variance). Non-additive genetic influences (D) were not modeled as there was no evidence for them from inspection of the twin correlations (Neale and Cardon 1992). Correlations in DZ twin and non-twin siblings, who share their genetic material to the same extent, were compared and differences in these correlations were attributed to an additional special twin environment parameter (T; sources of twin similarity that do not relate to non-twin sibling resemblance; e.g., sharing a classroom, in-utero effects).
