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If you're acting stupid because you're a stoner, you might just be playing to type. That is, it may be your expectations about marijuana's long-term cognitive effects — rather than any real effect of the drug itself — that is to blame, particularly if you're male, according to new research. The study, which was published in the journal Addictive Behaviors, explored the effect of "stereotype threat" — the idea that performance is affected by conventional images of minorities — on marijuana smokers.

Earlier studies of stereotype threat have found that when African Americans are asked to identify themselves by race before being tested, they tend to score worse than blacks who weren't reminded of their race — in line with racist stereotypes about blacks doing poorly in school.

Explains study co-author Mitch Earleywine, professor of psychology at the University of Albany–SUNY: "The bottom line is if you get folks into the lab and prime them about the minority group they're in, they tend to behave consistent with the stereotype. With Asian women, if you bring them in and say, 'Oh, you're a chick, you can't do math,' they tend to do lousy. But if you say, 'Oh, you're an Asian,' they do well."

Earleywine wondered if the same could be true of marijuana smokers. While there are obviously acute effects of the drug on memory when someone is high, the research on marijuana's long-term impact on cognition, when not under the influence, is complicated. A meta-analysis that summed up the research found no clinically significant effects, but some individual studies have shown small but measurable cognitive differences between long-term, short-term and non-users. Is it possible that the studies that found a negative effect had cued marijuana users to the fact that they weren't supposed to do well?

Earleywine and his colleagues studied 57 users, 30 male and 27 female. Half were given material to read suggesting that marijuana damages the brain; the other half read a research summary suggesting that the drug had no long-term negative cognitive effects. Then, all participants were asked to take cognitive tests after abstaining from marijuana for at least one day.

There was a marked difference in results — interestingly, between men and women. Men who got the negative information about marijuana performed worse than men who didn't, but the women who were faced with stereotype threat actually scored better on tests of verbal skills and memory than women who weren't given negative information.

"I think that the stoner identity reads differently to women," says Earleywine. "They were like, 'I'll show you!' It was funny to see that go in the opposite direction. They did better than those who read that [marijuana has] no impact, on three of four measures."

"It certainly seems plausible that if someone came up and told you that marijuana caused cognitive impairment and gave you some test that you might respond by showing some cognitive impairment," says Dr. Harrison Pope, professor of psychiatry at Harvard Medical School, who has conducted studies of marijuana and cognition himself. "But what is odd is that women would have paradoxically shown better performance whereas men showed worse. I'd like to see it replicated."

If the finding holds up, says Pope, "It would throw in yet another factor one has to consider when studying the effects of marijuana."

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"Casual" use is one 1970 vintage joint per day - not deemed physiologically harmful.

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