who reported at least a 4-week period of drinking ≥7 drinks per week (males) or ≥4 drinks per week (females). The second, termed ‘Perception of Alcohol Problem’, required a positive response to at least one of three questions: Has there ever been: (i) a time in your life when you drank too much? (ii) A period in your life when someone else objected to your drinking? (iii) A period in your life when you would drink instead of working or spending time with hobbies, family or friends? Subjects who were negative to all these screening criteria were skipped out of the alcohol section and no data were recorded on the presence of absence of individual AD criteria. The full information for maximum likelihood method implemented in Mx makes use of all the available twin information, and can provide asymptotically unbiased parameters when selection items are included and missingness can be assumed to be ‘at random.’26 Mx optimizations were performed using both the try hard option and different starting values to reduce the possibility that a solution found was local rather than global minimum.
