In the January 2008 issue of Reason, I compared trends in marijuana arrests to trends in marijuana use and concluded that "there is no clear relationship between the number of arrests and the number of pot smokers." That is, it did not seem to be the case that a) increases in use were driving up arrests or b) increases in arrests were driving down use. In the November issue of The Bulletin of Cannabis Reform, Jon Gettman takes a more detailed and sophisticated look at the numbers and reaches much the same conclusion:

The most important characteristic of marijuana arrests in the United States is that they have been steadily increasing over the last 20 years with little or no impact on the level of marijuana use in this country....Marijuana arrests have nearly doubled from 1991 to 2008, increasing by 150% during the 1990s and increasing steadily in recent years, producing an annualized change of 6.56% per year during this period. Overall, levels of marijuana use in the United States have remained fundamentally unchanged during this period.

The implication is that the risk of arrest has gone up. By my calculation, comparing annual arrests to annual users, it has roughly doubled. But Gettman argues that "the overall marijuana arrest rate of between 3% and 6% of users is not enough to represent a meaningful deterrent." He also notes that the risk is not evenly distributed:

While the marijuana-use rate for African-Americans is only about 25% greater than for whites, the marijuana possession arrest rate for blacks is three times greater. This is not a regional disparity, but is seen in every state and most counties.

Looking across jurisdictions, Getmann finds little evidence that relatively lenient treatment of pot smokers is associated with higher levels of use or that relatively harsh treatment is associated will lower levels of use. He estimates that "marijuana arrests cost state and local governments $10.3 billion in 2006."

The whole report is here (PDF). And in case that does not satisfy your appetite for marijuana arrest numbers, the Marijuana Policy Almanac, which includes "state rankings and individual reports for all 50 states plus the District of Columbia," is here.

I considered the latest marijuana arrest figures in September.

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See a state-by-state list of the effects of marijuana legislation.