This study is unique as it provides the opportunity to examine brain-based changes as a function of alcohol use initiation in the context of a longitudinal design where baseline use is absent, and premorbid characteristics are highly similar between non-users and initiators. Overall, findings cohere with an increasing body of evidence that normative developmental changes in cortical thickness, white matter extent, and white matter microstructure are impacted by the initiation of alcohol use in the mid-to-late adolescent period. Participants in the current study reported that they initiated regular alcohol use, typically in peer-related social contexts, between the ages of 17 and 18 on average. As indicated through many recent studies of adolescent brain development, mid-adolescence is an active phase of brain development characterized primarily by increases in white matter as well as evidence of microstructural changes in the directional organization of white matter, particularly in circuits that link the striatum and limbic regions with the frontal lobe (19,20,22). In contrast, although declines in cortical gray matter as well as cortical thickness are also observed, these changes are maximal early in
