What is the psychology behind trolling? For those new to the internet, trolling is an activity where one person intentionally tries to upset other members of the same community, presumably for entertainment. 

This has been informally addressed in the media, normally positing that the anonymity of the internet gives rise to more extreme behaviors, 


but is that the explanation research supports? 
And if it is, how does the process of enabling occur?
And more broadly, why do people derive pleasure from the anger of others within the same group?
Lastly, is this behavioral phenomenon unique to the internet era, or are there historical predecessors involving similar causal mechanisms?
There are a few references to the scientific literature on trolling in the wikipedia article


  Some psychologists have suggested that flaming would be caused by
  deindividuation or decreased self-evaluation: the anonymity of online
  postings would lead to disinhibition amongst individuals (Kiesler et al, 1984). Others
  have suggested that although flaming and trolling is often unpleasant,
  it may be a form of normative behavior that expresses the social
  identity of a certain user group (Lea et al, 1992; Postmes et al, 1998).


References


Kiesler, S. , J. Siegel and T.W. McGuire (1984). "Social psychological aspects of computer-mediated communication". American Psychologist 39 (10): 1123–1134. doi:10.1037/0003-066X.39.10.1123. LINK
Lea, M., T. O'Shea, P. Fung and R. Spears (1992). "'Flaming' in Computer-Mediated Communication: observation, explanations, implications". Contexts of Computer-Mediated Communication: 89–112.
Postmes, T., Spears, R., &amp; Lea, M. (1998). "Breaching or building social boundaries? SIDE-effects of computer-mediated communication". Communication Research (25): 689–715. FREE PDF