Our measure of availability may have indexed perceived rather than actual cannabis availability, and so it remains an empirical question as to which is a better predictor of initiation and progression to abuse. One study has shown that perceived availability was a significant predictor of cannabis, heroin, alcohol and tobacco use but not of LSD or non-prescribed tranquillizers (50). The large `C' component indicates a common environmental risk or set of risk factors shared between twins. Because the reporting period mostly spanned adolescence, it is possible that the `C' is capturing aspects of the twin environment provided by the twins' parents who may be more or less conducive to drug exposure and initiation by way of lack of parental supervision or parental attitudes towards drug use. Alternately, the `C' component in availability might reflect the general impact of neighbourhood, school or church effects, or the specific effects of correlated exposure to deviant peer groups or level of participation in pro-social activities (51, 52).
