From the NYTimes story on a recent study that found this interesting effect:
... People who washed their hands after contemplating an unethical act were less troubled by their thoughts than those who didn’t, the study found.
“The association between moral and physical purity has been taken for granted for so long that it was startling that no one had ever shown empirical evidence of it,” said Chen-Bo Zhong, an author of the new research and a behavioral researcher at the University of Toronto. The study, which he wrote with Katie Liljenquist, a graduate student at Northwestern University, appeared in the journal Science.
The researchers call this urge to clean up the “Macbeth effect,” after the scene in Shakespeare’s tragedy in which Lady Macbeth moans, “Out, damned spot! Out, I say!” after bloodying her hands when her husband, at her urging, murders King Duncan.