It is plausible, particularly with substance use such as alcohol use, that the behaviour of a twin may have an impact on the same behaviour in the other twin. This is termed ‘sibling interaction’ and can take two forms, sibling competition and sibling cooperation [34]. In traits measured on an ordinal scale we would expect a sibling interaction to result in differences in threshold levels and in the prevalence of individuals in extreme categories for MZ and DZ twins [37]. This can be tested by assessing whether the MZ and DZ twin thresholds can be equated without a significant drop in fit. We found that MZ and DZ thresholds could be equated for all variables, suggesting that there was no significant effect of sibling interaction (results available from the corresponding author upon request).
