Problematic alcohol use is associated with significant impairment in multiple domains of functioning, including deviations in brain structure and functioning. However, the causal basis of these associations remains unclear—problematic alcohol use appears to have neurotoxic effects on the brain, but it is also plausible that brain deviations reflect pre-existing liability toward problematic alcohol use in the first place. The Twin Brains study is a secondary data analysis project that uses differences in the amount of alcohol use in co-twins to differentiate pre-existing liability toward problematic alcohol use from exposure-related effects of alcohol on the brain in two large, population-based, genetically informative samples of ~1200 adult twins from the Minnesota Center for Twin and Family Research (MCTFR) and ~1200 adult twins and their nontwin siblings from the Human Connectome Project (HCP).