It turns out there's a psychological explanation for "trolling," the practice of leaving nasty, angry, inflammatory messages on message boards, blogs and other open-comment web sites:

Social psychologists have known for decades that, if we reduce our sense of our own identity -- a process called deindividuation -- we are less likely to stick to social norms…the same thing happens with online communication such as email.

It's a fascinating post; go read the whole thing.

Hat tip to Gavin Richardson.