Yahoo! News - Revenge Really Is Sweet, Study Shows: "'After squeezing back the intruder, you can't help but notice a smile creep onto your face,' Knutson wrote in a commentary.

That instinct probably evolved to grease the wheels of human social interaction, the researchers said.

'For thousands of years, human societies did not have the modern institutions of law enforcement -- impartial police and impartial judges that ensure the punishment of norm violations such as cheating in an economic exchange, for example,' they wrote.

'Thus, social norms had to be enforced by other measures, and private sanctions were one of these means.'"