Adolescent alcohol abuse and adverse adult outcomes: evaluating confounds with drinking-discordant twins.

BACKGROUND: Adolescent alcohol abuse is associated with adverse outcomes in early adulthood, but differences in familial status and structure and household and community environments correlate with both adolescent drinking and adverse adult outcomes and may explain their association. We studied drinking-discordant twin pairs to evaluate such confounds to ask: Will between-family associations replicate in within-family comparisons?
METHODS: With longitudinal data from >3,000 Finnish twins, we associated drinking problems at age 18½ with 13 outcomes assessed at age 25; included were sustained substance abuse, poor health, physical symptoms, early coital debut, multiple sexual partners, life dissatisfaction, truncated education, and financial problems. We assessed associations among twins as individuals with linear regression adjusted for correlated observations; within-family analyses of discordant twin pairs followed, comparing paired means for adult outcomes among co-twins discordant for adolescent problem drinking. Defining discordance by extreme scores on self-reported problem drinking at age 18½ permitted parallel analyses of twins as individuals and discordant twin pairs. Alternate definitions of pair-wise discordance and difference score correlations across the entire twin sample yielded supplementary analyses.
RESULTS: All individual associations were highly significant for all definitions of discordance we employed. Depending on definitions of discordance, 11 to 13 comparisons of all drinking-discordant twin pairs and 3 to 6 comparisons of discordant monozygotic (MZ) twin pairs replicated between-family associations. For most outcomes, effect size attenuated from individual-level analysis to that within discordant MZ twin pairs providing evidence of partial confounding in associations reported in earlier research. The exception was the General Health Questionnaire (GHQ); at age 25, GHQ-12 had equivalent associations with age 18½ Rutgers Alcohol Problem Index across all comparisons.
CONCLUSIONS: Our analyses control for shared family background, and, partly or fully, for shared genes, to yield within-family replications and more compelling evidence than previously available that adolescent alcohol abuse disrupts transitions into early adulthood.