From ages 19 to 21, the risk of criminality dropped steadily. There were somewhere between 21,000 and 22,000 charges filed against people exactly 19-years-old in the data set, the economists found. That number fell to about 19,000 charges filed against people just shy of 21.

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As soon as people turned 21, their likelihood of criminality spiked considerably. (The researchers did not adjust for population, but co-author Glen Waddell noted in an email that “population does not sharply change at 21.")

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The number of charges filed against 21-year-olds was similar to the number for 19-year-olds. In other words, from a criminal-justice standpoint, turning 21 is akin to turning back the clock to your late teens.

The mechanism by which this works is fairly obvious — access to alcohol increases dramatically at age 21. That brings more intoxication, and with it more aggressive, belligerent and criminally stupid behavior. Researchers Mark Kleiman, Jonathan Caulkins and Angela Hawken wrote in a book on drug policy in 2011 that “there is a good deal of evidence showing an association between alcohol intoxication and violent crime.”

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The relationship between the drinking age and the observed boost in crime is just an association, not necessarily a causal link. But the University of Oregon's Waddell noted in an email that there are not other plausible confounding factors that explain the crime spike. Kids in Oregon get drivers licenses at age 16, can drop out of school at age 17, and can vote and purchase handguns at age 18. There are no other big developmental or legal milestones at age 21 that would explain the shift.

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The Oregon economists dug into these numbers a bit deeper, teasing out which types of crime seem to jump at age 21 and which do not.

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“DUI’s, reckless driving, disorderly conduct, alcohol possession in parks, and selling to minors all increase with legal access to alcohol,” they wrote.

More troubling, turning 21 brings an uptick in the likelihood of committing assault, especially for people with no prior criminal history. For assault, the increase at age 21 for first-time offenders is 10 time as large as it is for those who have committed crimes. There is a similar disparity in charges for drunken driving, which are more likely to spike for those with no criminal record.

In other words, the researchers write, it is possible that “alcohol availability induces individuals into criminality, rather than increasing the criminality of those with established histories of violating the law.”