We used the raw ordinal analysis method in Mx (55) to analyze the twin data. This approach is based on the Central Limit Theorem which assumes that ordered categories reflect an imprecise measure of an underlying, normal liability distribution, and that this distribution has one or more threshold values which discriminate between categories (59, 60). All analyses were corrected for the linear effects of age at interview to remove age and cohort effects which are confounded in these data.
