I can only provide a single data point, but I used to smoke a lot of weed for a pretty long time, then stopped about 12 years ago, and one of the changes I noti

I can only provide a single data point, but I used to smoke a lot of weed for a pretty long time, then stopped about 12 years ago, and one of the changes I noticed was that I became much more able to accept responsibility for my mistakes instead of blaming everything wrong with my life on others. After I stopped smoking, I talked about this a friend who also quit weed around that time, and he said it was the same for him. (Okay, so it's two data points.) Make of that what you want, but there is a correlation. That doesn't mean weed turns people into assholes necessarily, but it can certainly amplify preexisting tendencies in that direction. I'm pretty sure the same goes for various other drugs as well - I never spent time around cocaine or methamphetamine enthusiasts, but I've been told, some people have a tendency to become real assholes on cocaine. And of course there's alcohol, which definitely helps all kinds of unpleasant personality traits to float to the surface.