The only reason weed is illegal is because it was cutting into the logging industry. When it was first made illegal there was 0 scientific proof of any negative effects (not saying there arent any but they had no proof), propaganda was put out claiming it was so horrible and everyone jumped on the bandwagon. Its similar to alchohol during prohibition, the government made up facts to scare the general public. Some of the propaganda from prohibition is still taken as fact to thia day, for example alchohol doesnt actually kill brain cells (binge drinking will but having a few drinks causes no permanent harm) what it does is block certain receptors in your brain which in turn affects your motor control, speech, and your ability to think quickly. This blocking of the receptors does make you temporarily dumber but the changes will revert once the alchohol has left your system, and unless you drink in excess for days on end the damage will only be temporary.