Generally speaking, potheads don't have a reputation for clarity of mind, but a new study finds that those who begin partaking when they're under 16 ar even more likely than late adopters to perform badly on cognitive tests. Staci Ann Gruber of Harvard Medical School, who conducted the research, presented her conclusions at Monday's annual meeting for the Society for Neuroscience. Here's the short version:

How did the study work?
Gruber recruited 35 "chronic marijuana smokers" and 29 non-smokers. Of the first group, 20 people began smoking before the age of 16, and the other afterward; 16 was chosen as a cutoff point, says Gruber, because "that's when teens tend to [begin to] use drugs." The participants (whose age averaged 22) were asked to perform a series of brain exercises such as the Wisconsin Card Sort Task — a cognitive test that involves categorizing cards according to shape, color, and other factors.

What did the study find?
The pot smokers performed worse than the control group — making repeated errors and showing an inability to maintain focus — and the "early-onset" smokers performed markedly worse than those who developed a marijuana habit after age 16. Gruber concluded that "early-onset smokers were not able to adjust their thinking on the fly or to focus their attention on a task." The study also found that kids who start smoking pot early consume three times as much, and twice as often, as kids who start later.

What are the ramifications of this?
Gruber believes early-onset smokers may "have trouble with abstract thinking" or be more likely to "say something socially inappropriate," and adds that the cognitive impairments appear to be "irreversible." Meanwhile, not everyone is convinced: "I’m a little suspicious," says Harvard Medical School associate professor emeritus Lester Grinspoon, who wants to revisit the conclusions "after [the study has been] peer reviewed."

Sources:, WebMD, Harvard Crimson