There was recently a study which showed 8 point decrease in IQ for kids smoking weed. (Article) If the study is accurate, I'd worry it becomes too easy to get for young people, like cigarettes or booze.
First, please don't distort the article you've posted. It says an 8 point drop for the "heaviest" smokers who started before 18. We're talking highschool kids puffing 10+ joints a day, or something. Smoking pot between classes. People like that do exist.
IQ is a really dubious measure, and it's also a chicken-and-egg thing. IQ is
trained by educational activities. Correlation is not necessarily causation. That article doesn't take into account that teens in environments where they're opening to becoming chronic pot smokers probably didn't have such great home or study environments in the first place. They actually admit that they didn't control for home-life or child-abuse history.
And since that 8 points is only the heaviest smokers, you have to consider that the kid has THC in his bloodstream all the way through class time, given how long it stays in your system.
So i'd dispute that this factor "decreased" their IQ. If I goof off and don't study for a test, thus getting a low score, did my intelligence drop?
Hell I used to smoke a lot of pot and I do maths + programming for fun, and score excellent on IQ tests, I stopped now because I was concerned about lung health and saving money.