Physical cleanliness and moral purity have a long association in religion, language and other human behaviors. Cleanliness is next to godliness; the Mandarin term for a thief is "a pair of dirty hands"; and, perhaps most famously, Lady Macbeth desperately attempts to wash away a spot of blood after murdering Duncan. Behavioral researchers Chen-Bo Zhong of the University of Toronto and Katie Liljenquist at Northwestern University explored this so-called "Macbeth effect" in a series of experiments with undergraduates. The research revealed that, unconsciously at least, you can wash away your sins.

In the first study, 60 Northwestern students were isolated and asked to describe either an ethical or unethical action they had undertaken in their lives. Following this exercise, they were presented with a series of six word fragments, three of which--W_ _ H, for example--could be completed in a cleansing way (WASH) or an unrelated way (WISH). Those who had just spent time recalling an unethical deed were more likely to produce a cleansing word. And a subsequent similar study with 32 subjects showed that after recalling an unethical memory, students were more likely to choose a free antiseptic wipe over a free pencil when offered the choice, as compared with untested subjects who showed little preference.

In another study, 27 subjects hand copied either an ethical or unethical story. In the ethical version a lawyer helps his colleague, whereas in the unethical version the lawyer sabotages him. Then the students rated products, including cleansing ones such as soap or toothpaste. As expected, the students who had copied the unethical story rated the cleansing products significantly more highly than their ethical peers.

Although these studies seemed to show that moral stains produce a desire for physical cleanliness, Zhong and Liljenquist wondered whether such a need to be clean could actually drive behavior. After asking 45 more students to recall an unethical behavior from their past, the researchers offered 22 of them a sanitary wipe while leaving the rest of their peers in an "unclean" state. They then asked for unpaid volunteers to aid a desperate graduate student with another study: 74 percent of those in the unclean state offered their help versus only 41 percent of those who had cleaned themselves, according to results published in the September 8 issue of Science. "Washing hands can reduce physical disgust but it can also reduce moral emotions," Zhong says.

Having discovered this unconscious association, Zhong and his colleagues hope to explore the roots of this link--whether in culture, language or the psyche--as well as its implications. "How does environmental cleanliness influence people's ethical behavior?" he asks. "What our study will try to demonstrate is that there might be a direct causal relationship from unclean environment to social behavior." Perhaps if Macbeth had helped his Lady keep a clean home, they might not have engaged in such dirty deeds.