father was not DD or AD. Offspring whose uncle was an MZ co-twin of their father were assigned to group D2 if the co-twin was DD, and to group A2 if the MZ uncle was AD (these offspring are at high genetic risk because their father is genetically identical to his co-twin, but reduced environmental risk since they were not raised by a substance dependent father). Offspring whose uncle was a DZ co-twin of their father were assigned to group D3 if the co-twin was DD, and to group A3 if the DZ uncle was AD (moderate genetic risk because their father shares approximately 50% of his segregating genes with his co-twin, but again reduced environmental risk). The comparison group for all analyses was offspring with no DD or AD diagnosis in either the father or the uncle (low genetic, low environmental risk). Because offspring genetic risk is inherited from both parents, maternal history of AD and heavy cannabis use (as an indicator of DD) were controlled for statistically, as described below.
