Researchers at the University of Toronto published data that suggests people experience "a powerful urge to wash themselves" when suffering from a guilty conscience. This urge is known as the "Macbeth effect," referring to Shakespeare's famous play in which one of the main characters cries, "Out, damned spot!" while trying to scrub away bloodstains that exist only in her mind.

In order to study this effect, the researchers asked volunteers to think about immoral acts they had committed in the past—shoplifting, betraying a friend, and so on. The volunteers were then offered an opportunity to clean their hands. According to the results of the study, those who had retraced their sins "jumped at the offer at twice the rate of study subjects who had not imagined past transgressions."

Interestingly, the act of washing did relieve the guilt of many volunteers—at least temporarily. After deciding whether or not to wash, the subjects who had felt guilty were given a chance to volunteer for a charity event. Those who actually washed their hands "were far less likely to sign up than those who didn't wash."