A co-twin-control analysis of adolescent and young adult drinking effects on learning and memory

Existing evidence for a link between alcohol use and memory impairments in adolescents and young adults is largely correlational. We aimed to determine whether associations between drinking and episodic memory were consistent with a causal effect of drinking or accounted for by familial factors confounding such associations. Because cannabis use is associated with a similar pattern of performance on episodic memory measures, we assessed whether any associations might be attributable to concurrent cannabis use.

Observational study of individuals aged approximately 20-29 years, comprising two independent population-based cohorts of twins. A co-twin-control design permitted an estimate of alcohol exposure effects free of shared genetic and environmental confounding influences. Significant associations were followed-up with twin-difference analyses. Propensity scores derived from measures collected at age 11 were used to adjust for unshared confounders. Participants in both cohorts were assessed from the age of 11 (n = 1251) under the auspices of the Minnesota Center for Twin and Family Research.

Regression analyses with cumulative alcohol use as the predictor of interest. Multiple measures of attention, learning and memory from a widely used episodic memory task constituted dependent variables.

Drinking was associated with poorer attention (P ≤ 0.003) and learning (P ≤ 0.008). Results were similar across the two cohorts. The within-pair effect in twin-difference analyses was significant only for measures of learning (P-values ≤ 0.004). Results were not due to measured unshared confounders or cannabis use. Drinking in adolescence (to age 20) and early adulthood (between 20 and 29) exerted independent effects on learning.

There appears to be a robust and specific association between drinking and learning that can be reproduced across cohorts, is not easily accounted for by confounding factors or concurrent cannabis use and is consistent with a causal influence of drinking.