Independent-pathway structural equation twin models were fitted using the full information by maximum likelihood method in Mx.25 We tested for both quantitative sex effects (that is, is the magnitude of genetic influences different in men and women?) and qualitative sex effects (that is, do the same genetic factors influence risk to AD in the two sexes?). To obtain unbiased population-based parameter estimates, the model had to take into account the structure of the interview through which individuals were either selected into or skipped out of the AD section. Two additional binary items were created to encode our AD screening procedure. The first criterion, termed ‘excess quantity/frequency,’ was coded positive for individuals who were skipped into the AD section because they admitted that they consumed on a single day at least 13 drinks (for males) or 7 drinks (for females); or 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
