Similar article on the same study: BBC NEWS | Health | Cannabis 'disrupts brain centre'

Although I realize that this was a very well-conducted study, and while I realize that, of course, there are risks associated with smoking pot, I still think the fear-mongering rhetoric that "THC levels are thought to have doubled in street cannabis in recent years" is bullshit.

First of all, the fact that potency has increased (which is probably true, just as crop yields of corn are increasing through selective mating and genetic engineering) simply means that people will smoke less plant matter for the same high, meaning less risk for cancer. Additionally, the position that THC has increased but cannabidiol (CBD, to use the BBC's abbreviation) has remained the same simply does not follow -- breeders breed for potency, not for THC levels at the expense of CBD levels. And finally, that cannabis use increases schizophrenic symptoms -- just like all other hallucinogens -- seems obvious; in my opinion, people with any form of illness should not use any drug without weighing the risks -- and there are, of course, risks -- against the benefits of the high (which, Purp, I'm sure you have done).

In short, while this study may be valid, experimentation should not be performed in order to prove what seems to me like a fairly clear fact; rather, as birdgirl said, studies should be performed to advance knowledge of this rather mysterious plant, not to bolster previous knowledge.