In a decade-long study of more than 1,000 New Zealanders, researchers found that adolescents who used marijuana at least four days a week lost an average of eight IQ points between the ages of 13 and 38, which is a pattern not seen among people who began smoking heavily only in adulthood.

Teenagers who smoke frequently may be setting themselves up for a decline in intelligence over the years, as reported by CNN.

Heavy pot smokers also tended to show deficits in memory, concentration and overall brainpower in relation to others, but these problems were more prounced among those who picked up the habit at younger ages.

“The effect of persistent cannabis use on intellectual functioning is really confined to adolescents, (which) suggests that adolescents, in particular, are vulnerable to the effect of cannabis,” says lead author Madeline H. Meier, a psychologist and postdoctoral researcher at Duke University in Durham, North Carolina.

Previous research suggests that a drop in intelligence of that magnitude could affect a person’s long-term career prospects, performance and income, as reported.

The study was published earlier this week in the Proceedings of the National Academy of Sciences. The study doesn’t prove marijuana to directly impair intelligence, but it does provide some of the strongest evidence to date of a cause-and-effect relationship.

The authors say that more research will be needed to determine the minimum dosage of marijuana associated with these problems.

What do you think of the study? Should we be focused on figuring out if it declines in IQ or should the study focus on a different cause-and-effect relationship? Do you think the marijuana laws should be changed in New York?