An article published recently in the Journal of Consumer Research finds that the physical appearance of money influences how people spend it.
In short, people tend to spend old, worn-looking bills faster than new, crisp bills. The authors speculate that old-looking bills are seen, at an unconscious level, as gross and “contaminated” by the hands of others.
One situation where people will spend newer bills more quickly? When they believe they are being watched by their peers.