Greg Campbell’s comparison of marijuana smokers to buyers of blood diamonds (h/t Andrew Sullivan) is hardly original: the Partnership for a Drug-Free America ran a series of ads on this theme a decade or more ago, one of them showing a young pot-smoker saying “I killed a judge today.” But it comes wrapped in two spectacular fallacies.

First, Campbell cites unnamed ”Mexican officials” for the claim that “cannabis now provides the cartels with as much as half of their revenue.” That’s down from the zombie estimate of 60% that U.S. officials continued to prop up until Beau Kilmer, Jonathan Caulkins, Brittany Bond, and Peter Reuter at RAND drove an analytic stake through its heart, but it’s still nonsense; RAND estimates the true figure at 16-26%. (Link goes to a .pdf; see p. 33.) And that’s the marijuana share of their drug-export revenues alone, ignoring all the money they make from extortion, kidnapping, theft from PEMEX, and drug sales to Mexican consumers.

Second, Campbell identifies “the casual marijuana smoker” as the class of person with the “responsibility for ethical behavior.” But of course casual smokers don’t, by definition, smoke very much. Marijuana consumption, like alcohol consumption, obeys the 80/20 rule (sometimes known as “Pareto’s Law”) under which 20% of the people engaged in any activity account for 80% of the activity. So Campbell should be addressing his message to serious pot-heads rather than Saturday-night tokers.

But the sort of serious pot-head who reads the New Republic and worries about the impact of his consumer behavior on violence abroad probably smokes expensive sinsemilla – mostly domestic or Canadian in origin – rather than cheap Mexican product.

And those prosperous, literate, socially conscious folks constitute a distinct minority among heavy marijuana users; 62% of the days of marijuana intoxication are accounted for by users with a high-school education or less. The social gradient associated with cannabis use has reversed since the 1960s, but that fact has yet to fully penetrate the public discourse ; as Jon Caulkins has remarked, marijuana is now predominantly used by Wal-Mart customers rather than Whole Foods customers.

Campbell notices the fact that legalization would end the problem only to dismiss it with the claim that “legalization isn’t likely to happen anytime soon.” But that’s not true at the state level: with three propositions on the ballot this fall (Colorado, Oregon, and Washington) it’s probably better than even money that at least one will pass, and – unless that state, or the federal government, stepped in with massive enforcement – a single state that legalized outside the medical context could easily supply the entire continent with pot, more or less wiping out the Mexican cannabis trade. Maybe that’s not a sufficient argument for legalization, but it’s substantial one.

So the logical conclusion of Campbell’s argument would seem to be that pot-smokers concerned about the impact of their habit on Mexican violence should put aside some of their budget for marijuana purchases to contribute to one of the legalization efforts. That’s not quite the conclusion he draws.

[Cross-posted at The Reality-based Community]

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Mark Kleiman is a professor of public policy at the New York University Marron Institute.