http://www.wired.com/wiredscience/2011/08/does-marijuana-make-you-stu...

Marijuana is currently regulated by the United States government as a Schedule II drug, placing it in the same category as methadone, cocaine and morphine. This is largely due to the first condition of Schedule II drugs, which is that the substance “has a high potential for abuse.” The language in that clause is deliberately vague. Does abuse equal addiction? Probably not, since marijuana is not addictive like other Schedule II drugs. Rats don’t self-administer the compound in a lab, it’s virtually impossible to fatally overdose on the drug, and the physiological effects of marijuana withdrawal, if they occur, are far milder than those experienced by chronic amphetamine, alcohol, nicotine or opiate users. Put another way, if “abuse” means “addiction” then cigarettes should be Schedule II, not marijuana.

Rather, the case for marijuana “abuse” has always stemmed from its cognitive effects. While cigarettes are like caffeinated smoke — they increase attention and productivity, marijuana is the drug of choice for slackers, hippies and Seth Rogen characters. In popular culture, all it takes is one hit from a bong before people become ridiculously dumb, unable to solve the simplest problems or utter a coherent sentence. Potheads eat a lot and laugh at stupid jokes. The larger worry, of course, is that such damage is enduring and that “smoking dope” permanently impedes learning and memory.

That, at least, has been the collective stereotype for decades. There’s even been some science to back it up, especially when the marijuana use begins at an early age. But now a different answer is beginning to emerge, thanks to an authoritative new study led by Robert Tait at the Australian National University. The scientists looked at the long-term cognitive effects of marijuana use in nearly 2,000 subjects between the ages of 20 and 24. The subjects were divided (based on self-reports) into several different categories, from total abstainers (n = 420) to “current light users” (n = 71) to “former heavy users” (n = 60). Over the course of eight years, the scientists gave the subjects a battery of standard cognitive tests, most of which focused on working memory, verbal memory and intelligence. One of the important advantages of this study is that the scientists controlled for a number of relevant variables, such as education and gender. In Time, Maia Szalavitz explains why this statistical adjustment is necessary:

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