Mono- (MZ) versus dizygosity (DZ) of twins was determined by a combination of questionnaire items and genotyping, resulting in a less than 1% miss-classification rate, which is unlikely to substantially bias results [34]. Our biometric analyses used data from the both waves in the same analysis. Only 16 twin pairs lacked all data relevant to biometric analyses and were excluded, plus one case with a recorded negative age of alcohol use onset. The 433 pairs with some information missing from either wave were retained in full-information maximum likelihood analyses [35], totaling in 1394 available pairs (Table 1 footnote for breakdown by zygosity and sex).
