I don't have proper sources, nor am I an expert on the subject. I just remember this being the result of a study I read about some years ago, and it fits my per

I don't have proper sources, nor am I an expert on the subject. I just remember this being the result of a study I read about some years ago, and it fits my personal experience with several friends. Some Googling finds this: Pope HG, Gruber AJ, Hudson JI, Huestis MA, Yurgelun-Todd D. Neuropsychological performance in long-term cannabis users. Arch Gen Psychiatry 58(10):909–915, 2001. Your caveat about correlation and self-medication could be right. It might also be wrong - you don't give a good citation either. My point was that I think we have a lot less knowledge about the effects of these drugs than e.g. alcohol which we're (ab-)using since millenia, so the UK analysis is certainly an improvement.