Mean monthly alcohol consumption, prior to transformation, ranged from 0.67 (SD=5.35) drinks at ages 12–14 to 39.55 (SD=75.66) at ages 22–25. After transformation, the total phenotypic variance of SELF1–4 increased from 0.27 to 2.84 between ages 12–14 and ages 22–25. The variance of PEER1–4 increased from 0.85 to 1.29. Phenotypic twin correlations (Supplementary Table 1) were similar for MZ and DZ twins at age 12–14, but at older ages, MZ correlations were modestly to substantially higher than DZ correlations. This was the case for both within-phenotype and cross-phenotype correlations, suggesting that genetic factors become increasingly relevant to phenotypic covariation over time.
