How expectations shape experience explored at book length in Mind Over Mind–and at interview length here (Scientific American):
[P]lacebo effects in medicine are just one example of how our expectations can bend reality. For instance, brain scans reveal that expectations about a wine’s quality (based on price or a critic’s review) actually change the level of activity in the brain’s reward centers when a person takes a sip. Highly-trained weight lifters can out-do their personal bests when they believe they’ve taken a performance booster. People who wear taller, better looking avatars in virtual reality behave in ways that taller and better looking people tend to act. For example, they approach better-looking potential dates and they are more aggressive in negotiations, both in the virtual world and after the headgear is removed. In lab and field experiments, people who stand in powerful poses (think Superman) for a minute or two, have similar hormonal changes to people who are given actual power and authority over another person, and they exhibit the same sorts of behavioral changes.
[via sullivan]