In the classic Milgram Experiment conducted in the 1960s, volunteers were told by an authority figure to deliver electric shocks to another person as punishment for incorrect answers to a test. The other person wasn't really receiving the shocks, but the volunteers were tricked into thinking they were by shouts of pain and protest. Despite this feedback, some volunteers went on to deliver what would have been lethal shocks.
Professor Mel Slater of the Catalan Polytechnic University has recreated the Milgram experiment using a computer simulated woman, with some interesting results. "The main conclusion of our study is that humans tend to respond realistically at subjective, physiological, and behavioural levels in interaction with virtual characters notwithstanding their cognitive certainty that they are not real." Some part of the brain just doesn't know about virtual reality.