I saw this program on the news yesterday, and it said that drug addicts have a hard time giving up the habit because their frontal cortex is impaired. The cortex supposedly has the effect of controlling impulses, but after taking drugs it stops working efficiently. I thought this is somewhat relevant to explaining negative thinking, and what could be done to fix it.
Here's the link to an online article about this research:
http://abc.net.au/news/stories/2007/...tion=australia