"Altruistic punishment": the rationalizing animal and the internet

From Amanda Knox: She was acquitted of the Meredith Kercher murder. Why do people still hate her so much?:

"Experiments show that when some people punish others, the reward part of their brain lights up like a Christmas tree. It turns out we humans avidly engage in something anthropologists call “altruistic punishment.” 

What is altruistic punishment? It is when a person punishes someone who has done nothing against them personally but has violated what they perceive to be the norms o...