About Rationally Speaking

Rationally Speaking is a blog maintained by Prof. Massimo Pigliucci, a philosopher at the City University of New York. The blog reflects the Enlightenment figure Marquis de Condorcet's idea of what a public intellectual (yes, we know, that's such a bad word) ought to be: someone who devotes himself to "the tracking down of prejudices in the hiding places where priests, the schools, the government, and all long-established institutions had gathered and protected them." You're welcome. Please notice that the contents of this blog can be reprinted under the standard Creative Commons license.

Wednesday, October 04, 2006

Dirty conscience? Get a bar of soap!

A truly bizarre study on human emotions has recently been published in Science (8 September 2006). The authors, Chen-Bo Zhong and Katie Liljenquist, set out to test the scope of the so-called “Macbeth effect.” The more literati among you may remember that in the Shakespearian tragedy, Lady Macbeth seeks to cleanse herself up after murdering King Duncan, attempting to substitute the physical washing of the hands for the washing of her sin. Zhong and Liljenquist noted that physical cleansing is part and parcel of almost every religious ritual (think of Christianity's baptism, or of Muslim”wudu,” a word meaning ablution). They therefore carried out a series of clever experiments aimed at quantifying the emotional effect of physical cleansing on one's conscience. The results were spectacular to say the least, and quite a bit disturbing.

In the first of four experiments, Zhong and Liljenquist had their subjects recalling instances of ethical or unethical behavior they had been involved in the past. They then asked them to complete words hinted at by providing a few letters. The test was set up so that completed words could be either neutral or “cleansing-related,” like 'wash,' 'shower' or 'soap.' The result was clear: recalling an unethical deed significantly increased the (unconscious) tendency to use cleansing-related words.

The second experiment involved copying a story given to the subjects, where the story could recount either a selfless or a selfish act by the protagonist. Subjects were then asked to rate a series of products, some “morally neutral” (like post-it) and some involved in physical cleansing (like Lysol disinfectant). Once again, the results were sharp: while copying ethically positive stories had no effect on product rating, after copying stories featuring unethical behavior people showed a dramatic preference for cleansing products.

A third experiment carried out by Zhong and Liljenquist tested the idea that – after recalling an unethical behavior of their own – people would be more likely to accept an antiseptic wipe than a neutral object, like a pencil. Sure enough, that's exactly what happened!

Lastly, and perhaps most remarkably, the researchers investigated the effect of physical cleansing on the tendency of people to “atone” or make up for their not so ethical behavior by engaging in activities perceived as having moral value. For example, people tend to donate to charities, or volunteer their time for good causes, after having engaged in a behavior they don't feel particularly proud of, essentially a way to re-establish one's “moral balance.” So, after having the subjects describe an unethical behavior from their own past, Zhong and Liljenquist asked them if they would volunteer their free time for a good cause (helping a desperate graduate student). The results were astounding: while 74% of control subjects agreed to volunteer, the percentage plummeted to 41% if people had been given the opportunity of, literally, washing their hands with sanitation wipes!

It appears that we have a strong (if entirely illogical) tendency to associate physical with moral cleanliness, something that religions the world over have probably picked up on and culturally reinforced. And of course the study by Zhong and Liljenquist also elegantly accounts for the puzzling behavior of Lady Macbeth in the Bard's immortal work.


  1. With regard to the fourth point, I am reminded of a Bible passage:

    "Your sacrifices are an abomination to me. The only sacrifice I require is a pure and upright heart." [only my paraphrase; Isaiah? Jeremiah?]

    One of those instances where tje Bible really is an enlightened document.

  2. It doesn't seem all that puzzling to me. We avoid bad behavior because of feelings of revulsion. We are disgusted when people do imoral things.

    It's reasonable to think that the same parts of our brains that cause us to avoid uncleanliness cause us to avoid immoral behavior. So the remedy for uncleanliness in the literal sense helps relieve our feelings of moral uncleanliness.

    We are an evolved system of co-opted mechanisms. Just as a birds wings are made by modifying a dinosaur's forelimb, our moral behavior is evolved from mechanisms that avoid unhealthy, dirty things.


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