The Louisville researchers noticed that dry counties had higher rates of meth lab busts, as well as higher rates of meth crimes overall. And the effect is significant: "if all counties were to become wet, the total number of meth lab seizures in Kentucky would decline by about 25 percent," they found.

After running some statistical tests, the researchers found that this is more than just a simple correlation: "Our results add support to the idea that prohibiting the sale of alcohol flattens the punishment gradient, lowering the relative cost of participating in the market for illegal drugs," they conclude.

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In other words: people who buy alcohol in places where it's illegal become accustomed to dealing with the black market. If you're going to get punished whether you trade in booze or trade in meth, why not give meth a spin? "Alcohol prohibition becomes a gateway to other illegal activities," as Tyler Cowen Alex Tabarrok sums it up at Marginal Revolution.

This research fits in with other findings showing harmful effects of localized alcohol prohibitions. A 2005 paper in the Journal of Law and Economics found that when Texas counties changed from dry to wet, their incidences of drug-related mortality decreased by 14 percent as people substituted alcohol for other drugs. Records from the Kentucky State Police show that dry counties tend to have higher rates of DUI-related car crashes than wet ones -- presumably because when you live in a dry county, you have to drive farther to get your booze. A 2010 report from the Robert Wood Johnson Foundation found that binge drinking rates were often higher in Alabama's dry counties than its wet ones.

There's no question that alcohol is one of the most dangerous widely-available drugs on the market today. But findings like the ones above suggest that the best way to manage the risks of alcohol consumption is to regulate it thoroughly and thoughtfully, not ban it outright.

There may be a lesson here for marijuana legalizers and their opponents. Colorado and other states experimenting with legalization are generally giving counties and municipalities the option to allow or prohibit marijuana sales within their own borders. While there's a case that's a sensible approach, the lessons of county-level alcohol bans suggest that in the long run, local governments that ban marijuana sales may simply be inviting a more insidious set of problems.