The third interview wave, or “MM3” (1998–2004), was completed solely by members of male-male twin pairs. Individuals were eligible for the MM3 if they came from a male-male pair, and if both had been interviewed in wave 2. Excluding triplets, interviews were completed by 1772 twins, aged 24 to 62 years (m = 40.3, SD = 9.0), representing a response rate of 74.7%. This included 459 monozygotic (MZ) and 282 dizygotic (DZ) twin pairs and 290 incomplete twin pairs or singletons (MZ=155, DZ=136). This interview included an assessment of perceived drug availability for cannabis for five age periods: 8 to 11, 12 to 14, 15 to 17, 18 to 21, and ages 21 to 25. We used a life history calendar method to increase the validity of our retrospectively collected data (30–32). This method has shown that although human memory is relatively poor at recall, it can be improved significantly when probed with careful questioning involving specific time periods and events. For cannabis, subjects were asked, “When you were [AGE] how easy would it have been to get CANNABIS if
