Yeah, if one individual has suffer it everyone else is in danger (false statement).
The same I could say for most people I know. All my teacher in the University smoke weed before I did born, all my classmate do, etc. And most of them doesn't show an intelect less than the average.
The deal is also that you are not telling all the details (violin). Because of course someone is going to become extra dumb if he doesn't read, doesn't study, etc. And all this is appart of smoking weed. So we shouldn't blame weed because he was a stupid kid who didn't took the chance of a good education.
Until know I haven't see any case of a person made dumb by weed (I have seen dumb people taking bad desitions, but not because weed did it).