Alcohol exposure is adolescence/early adulthood predicts adverse outcomes later in life. But the persistence and causal nature of these associations remain uncertain. We address those uncertainties with a 40-year follow-up of drinking-discordant monozygotic twin pairs assessed across multiple occasions from 1975-2016.
From the population-based Older Finnish Twin Cohort, we identified 171 pairs, born 1950-57, known or presumed to be MZ and meeting criteria for drinking-discordance in 1975 when ages 18-25. At baseline, the mean monthly consumption reported by the heavier-drinking twins in these discordant pairs was 556 grams/month versus 170 grams/month for co-twins. Twins in all pairs completed a postal questionnaire in 2011-12, and in 113 pairs, both twins completed a semi-structured telephone interview during 2014-16.
Results confirm significant long-term continuity of both individual and intra-pair differences in alcohol exposure with associated intra-pair differences in measured outcomes 40 years later. Individual differences in consumption correlated about 0.44 in both twin brothers and sisters across a 37-year span, and within-pairs, the heavier drinking twin in 1975 likely remained the heavier drinking twin on follow-up (p<.018). Scores on a Nordic questionnaire screen for alcohol problems were significantly higher among co-twins who reported heavier drinking decades earlier. Heavier-drinking co-twins reported significantly more alcohol disorder symptoms on interview (t = 2.78, p<.009), significantly lower scores on a Satisfaction with Life Scale (t = 1.86, p<.034), and significantly higher scores indicative of poorer mental health on the 12-item General Health Questionnaire (t = 1.96, p<.025). The correlation of signed intra-pair differences in consumption reported in 1975 with the number of alcohol disorder symptoms tallied from interviews in 2014-16 was highly significant (r = 0.26, p<.003).
Controlling for familial/genetic confounds, these results are consistent with inferences that heavy alcohol exposure in late adolescence and early adulthood causes adverse consequences across life course development. Those results encourage studies of epigenetic changes and paired comparisons of brain scans in this informative sample of drinking-discordant Finnish MZ twins.
