Hand washing: purifying the soul since 30 A.D.

Those of you familiar with the Bible will remember the famous words uttered by Pontius Pilate, the judge at Jesus’ trial, as he washed his hands in front of the crowd: “I am innocent of this man’s blood; you will see.”

While I have no intention to change my name to Religious Chick, I just wanted to point out that the link between physical and moral cleanliness goes back, waaay back. In a fascinating recent study published in the journal Science, researchers attempt to investigate an aspect of this link.

To better understand the study, you must first know the word(s) of the day: postdecisional dissonance. When you have to make a choice between two options that seem equally good (say, having lobster for dinner versus having King crab for dinner), you enter a state called “cognitive dissonance”, which essentially means that “I can’t decide!” feeling. In order to get rid of this uncomfortable feeling once you made your choice (say, crab), your mind does this funny trick where it now perceives your choice (the crab) as way better than the alternative you rejected (the lobster). This happens even though initially, you thought crab and lobster were equally attractive. This little mind trick is called postdecisional dissonance, and it’s been well documented. In the recent study, the researchers wanted to find out if washing your hands can affect this postdecisional dissonance mind trick.

The researchers set-up a mock consumer survey where they asked 40 subjects to chose, out of 30 CDs, 10 they would like to own. To thank them for participating in the “survey”, the subjects were offered a CD, either their 5th or 6th ranked selection. After they made their choice, the subjects were made to believe that they would now participate in a survey on liquid soap: they could evaluate it anyway they seemed fit. About half the subjects only looked the bottle, while the other half tested the soap by washing their hands. Finally, the participants were asked to rank the 10 CDs again, allegedly because the company behind the CD study wanted to know what people thought of the CDs before they left the store.

Using this (somewhat complicated) study design, the researchers were able to test if washing your hands can attenuate the need to justify a recent choice. The subjects who only looked at the soap ended up ranking the CD they chose in the first part of the experiment better (closer to 1 than 10). Their mind successfully played its trick where it convinced them that the choice they made was much better than the alternative, and this confirms the standard dissonance effect I describe above. Interestingly, the subjects who washed their hands ranked the CDs the same as how they had ranked it initially.

The researchers suggest that hand washing cleans us from past decisions, and reduces the need to justify them. It seems very strange to me that a common saying (“I wash my hands of what we have for dinner, lobster or crab, it’s all the same!”) and biblical anecdote are actually biologically grounded. I guess I shouldn’t be so surprised. What do you think? Can you come up with other examples?

A post from Julie Robillard, aka Scientific Chick.

Reference: Washing away postdecisional dissonance (2010) Lee SWS and Schwarz N, Science 328:709.

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