Note that, 75.2% of males and 64.7% of females reported lifetime use of cannabis. When stratifying by complete/incomplete twin pairs, cannabis use was elevated in female twins whose MZ co-twin or DZ opposite sex co-twin did not participate. While 59.8% and 64.0% of females from complete MZF and DZOS pairs reported lifetime cannabis use, respectively, 68.9% and 75.9% of singleton female twins from these zygosity groups reported lifetime cannabis use (p < .05). This pattern is expected if the variable in question is affecting compliance rate; fortunately FIML corrects for this ascertainment bias to give good estimates for the full population (Neale & Cardon, 1992; Neale et al., 1994). No other differences across zygosity groups were noted. While many individuals reporting lifetime cannabis use had used the drug relatively infrequently, prolonged, frequent, and problematic cannabis use was also relatively common in this cohort: as shown in Table 4: 23.0% of men and 11.3% of women reported using cannabis at least 100 times, 14.5% of men and 7.1% of women reported using it daily during their period of heaviest use, 23.3%
