This work has received some notice, and I thought it worth abstracting the clever experiments involved.
Edited from Zhong et al.: Physical cleansing has been a focal element in religious ceremonies for thousands of years. The prevalence of this practice suggests a psychological association between bodily purity and moral purity. In three studies, the authors explored what they call the "Macbeth effect"—that is, a threat to one's moral purity induces the need to cleanse oneself. In three studies they let this effect reveal itself through an increased mental accessibility of cleansing-related concepts, a greater desire for cleansing products, and a greater likelihood of taking antiseptic wipes.
In the first study they asked participants to recall in detail either an ethical or unethical deed from their past and to describe any feelings or emotions they experienced. Then they engaged in a word completion task in which they converted word fragments into meaningful words. Of the six word fragments, three (W _ _ H, SH _ _ ER, and S _ _ P) could be completed as cleansing-related words (wash, shower, and soap) or as unrelated words (e.g., wish, shaker, and step). Participants who recalled an unethical deed generated more cleansing-related words than those who recalled an ethical deed, suggesting that unethical behavior enhances the accessibility of cleansing-related concepts
The second study investigated whether an implicit threat to moral purity produces a psychological desire for cleansing, through expressed preferences for cleansing products. Participants were told that the experiment was investigating the relationship between handwriting and personality and were asked to hand-copy a short story written in the first person. The story described either an ethical, selfless deed (helping a co-worker) or an unethical act (sabotaging a co-worker). Participants then rated the desirability of various products from 1 (completely undesirable) to 7 (completely desirable). Cleansing products included Dove shower soap, Crest toothpaste, Windex cleaner, Lysol disinfectant, and Tide detergent; other products included Post-it Notes, Nantucket Nectars juice, Energizer batteries, Sony CD cases, and Snickers bars. Copying the unethical story increased the desirability of cleansing products as compared to copying the ethical story, with no differences between conditions for the noncleansing products
In the final study participants described an unethical deed from their past (the same recall task as in the first study). Afterwards, they either cleansed their hands with an antiseptic wipe or not. Then they completed a survey regarding their current emotional state. After completing the survey, participants were asked if they would volunteer without pay for another research study to help out a desperate graduate student. Presumably, participants who had cleansed their hands before being solicited for help would be less motivated to volunteer because the sanitation wipes had already washed away their moral stains and restored a suitable moral self.
As predicted, physical cleansing significantly reduced volunteerism: 74% of those in the not-cleansed condition offered help, whereas only 41% of participants who had a chance to cleanse their hands offered help. Thus, the direct compensatory behavior (i.e., volunteering) dropped by almost 50% when participants had a chance to physically cleanse after recalling an unethical behavior.
Thus, the authors showed that physical cleansing alleviates the upsetting consequences of unethical behavior and reduces threats to one's moral self-image. Daily hygiene routines such as washing hands, as simple and benign as they might seem, can deliver a powerful antidote to threatened morality, enabling people to truly wash away their sins.