As with genetic influences, environmental factors influencing lifetime co-occurrence of cannabis and tobacco use may be common to a general externalizing domain, to all forms of drug use or to the co-use of cannabis and tobacco alone. Twin studies identify two sources of environmental variation and covariation in cannabis and tobacco use. The first, which is more prominent during early adolescence, includes those environmental factors that are shared by members of a twin pair – these may include familial factors (e.g. parental monitoring) that both twins are exposed to or differing environments (e.g. recreational activities) that are perceived by both twins in a similar manner. These shared environments are distinguished from the second, which is individual-specific environment (e.g. life events experienced by one twin but not the other). Individual-specific environmental exposures are uncorrelated across members of a twin pair and are found to be highly significant contributors to cannabis and tobacco use and their lifetime co-occurring use across adolescence and adulthood (34). Like genetic influences, environmental influences on cannabis and tobacco use may be broadly classified as those that impact general externalizing behaviors, those that impact all forms of drug involvement and those specifically linking cannabis and tobacco use.
