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.

This is all you need to know to know that the author of the Forbes piece doesn't have any idea what he's talking about. Or, rather, that he's trying really hard to sell his book about EQ. He says, "They didn't measure this, but it's very important, nonetheless." Really? Why? Because you say so? Higher cognition and emotional control have very different neural correlates. Affecting one (if there is an effect; the PNAS article isn't that convincing to begin with), doesn't say anything about the other. That would have to be a separate study. So in the end, I'm not sure about the IQ decline associated with marijuana (my guess is that it's a little of column A and a little of column B), but I'm damn sure that Forbes is a bunch of used car salesmen quality hacks.