In a study of a large state university, students were randomly assigned roommates. researchers found that, on average, males assigned to roommates who reported drinking alcohol in the year before entering college had gpas one-quarter point lower than those assigned to non-drinking roommates

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We find that on average, males assigned to roommates who reported drinking in the year before entering college had one quarter-point lower GPA than those assigned to non-drinking roommates. The 10th percentile of their college GPA is half a point lower than among males assigned roommates who don't drink. For males who drank frequently before college, assignment to a roommate who drank often before college reduces GPA by two-thirds of a point