Though it is certainly not the only factor, research repeatedly finds that price affects consumption of psychoactive and other drugs, even among the heaviest users. A 2007 study in Health Policy found that a 10% increase on cigarette taxes would cut tobacco consumption by 3% to 5%. Other data suggest that raising taxes reliably increases quit-smoking attempts, successful quitting and reduction in use by those who don't quit — and that's in users of nicotine, which is more than twice as addictive as marijuana.

"We know from the alcohol and tobacco literature that higher prices are a big disincentive for people using drugs," says Dr. Evan Wood, founder of the International Centre for Science in Drug Policy, who was the lead author on a recent report on marijuana regulation.

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