With respect to individual-specific environmental influences, the degree of overlap across use and dependence symptoms was considerably higher for cannabis than for alcohol: non-shared environmental factors were correlated at 0.889 for cannabis vs. 0.546 for alcohol. Given that non-shared environmental sources of variance include error, interpretations must be made with caution, but the higher strength of the association for the illicit drug raises some interesting possibilities. For example, it could be argued that use of a common licit drug is normative and is therefore shaped by different environmental factors than pathological use, whereas use of an illicit drug is somewhat more deviant and access is more limited, so the role of environmental factors in use and misuse of an illicit drug may not be as distinct.
