Marijuana smokers for a long time have been characterised as dimwitted and slow. They tend to shrug off these stereotypes as artifacts of how they are when they’re on the drug, not how they are as people. If you’ve ever had the misfortune of enduring a pot smoker who takes you through the "beneficial” effects of marijuana on the brain, then you’ve likely wondered if the stereotype is true. As it turns out, it is.A new study more than thirty years in the making found that smoking marijuana permanently lowers intelligence, or IQ.  Pot smokers (even those who had given up marijuana) had deficits in memory, concentration, and overall IQ. The reduction in IQ for those who smoked pot heavily prior to age 18 was most pronounced - an average of eight points. An eight-point reduction in IQ is enough to have a significant, negative impact on your life. The significant thing about this research is that it was a longitudinal study: researchers followed and tested subjects from birth through to age 38, noting when and how frequently they picked up habits like drug use. Previous studies scrutinized marijuana use at a single point in time, which failed to eliminate the possibility that people with lower IQs are more likely to smoke pot. The longitudinal research provided a baseline IQ score for all subjects, which revealed changes in IQ scores as they picked up new habits, such as smoking pot.By following subjects for decades, the researchers were able to measure the lasting effects of marijuana use (even after subjects gave up smoking pot). These effects last because the teenage brain is still developing at a rapid pace; myriad new pathways for thinking are formed during this period while others are weeded out. When teenagers expose their brains to a damaging substance like marijuana, the effects aren’t just drastic…they’re permanently etched into the brain. Indeed, the reduction in IQ from smoking marijuana regularly was much greater for those who started smoking as teenagers than those who started in adulthood.While the study didn’t measure the effects of marijuana upon teenagers’ emotional intelligence, it’s likely they are dire. Emotional intelligence (EQ) in teenagers lags behind their cognitive development. This explains why teenagers are so impulsive, emotional, and prone to risky behavior. Since teenagers’ EQ develops much later than their IQ, this area of the brain is even more susceptible to the negative influences of marijuana.Forbes