CATHOLIC theology says that heaven awaits the pure of heart while hell is reserved for unrepentant sinners. For the sinful but penitent middle, however, there is the option of purgatory—a bit of fiery cleansing before they are admitted to eternal bliss. Nor is inflicting pain to achieve purification restricted to the afterlife. Self-flagellation is reckoned by many here on Earth to be, literally, good for the soul.

Surprisingly, the idea that experiencing pain reduces feelings of guilt has never been put to a proper scientific test. To try to correct that, Brock Bastian of the University of Queensland, in Australia, recruited a group of undergraduates for what he told them was a study of mental acuity.

At the start of the study, 39 of the participants were asked to write, for 15 minutes, about a time when they had behaved unethically. This sort of exercise is an established way of priming people with feelings associated with the subject written about. As a control, the other 23 wrote about an everyday interaction that they had had with someone the day before.

After the writing, all 62 participants completed a questionnaire on how they felt at that specific moment. This measured, among other things, feelings of guilt on a scale from one (very slightly guilty or not at all) to five (extremely guilty).

Participants were then told they were needed to help out with a different experiment, associated with physical acuity. The 23 who had written about everyday interactions and 20 of the 39 who had written about behaving unethically were asked to submerge their non-dominant hand (ie, left, if they were right-handed, and vice versa) into a bucket of ice for as long as they could. The remaining 19 were asked to submerge their non-dominant hand into a bucket of warm water for 90 seconds, while moving paper clips one at a time between two boxes, to keep up the illusion of the task being related to physical capabilities. That done, participants were presented with the same series of questions again, and asked to answer them a second time. Then, before they left, they were asked to rate on a scale of zero (no hurt) to five (hurts worst) how much pain they experienced in the warm water and the ice.

Dr Bastian reports in Psychological Science that those who wrote about immoral behaviour exposed themselves to the ice for an average of 86.7 seconds whereas those who had written about everyday experiences exposed themselves for an average of only 64.4. The guilty, then, either sought pain out or were inured to it. That they sought it out is suggested by the pain ratings people reported. Those who had written about immoral behaviour rated the ice-bucket experience at an average of 2.8 on the pain scale. The others rated it at 1.9. (Warm water was rated 0.1 by those who experienced it.)

Furthermore, the pain was, indeed, cathartic. Those who had been primed to feel guilty and who were subjected to the ice bucket showed initial and follow-up guilt scores averaging 2.5 and 1.1 respectively. By contrast, the “non-guilty” participants who had been subjected to the ice bucket showed scores averaging 1.3 and 1.2—almost no difference, and almost identical to the post-catharsis scores of the “guilty”. The third group, the guilt-primed participants who had been exposed to the warm bucket and paper clips, showed scores averaging 2.2 and 1.5. That was a drop, but not to the guilt-free level enjoyed by those who had undergone trial by ice.

Guilt, then, seems to behave in the laboratory as theologians have long claimed it should. It has a powerful effect on willingness to tolerate pain. And it can be assuaged by such pain. Atonement hurts. But it seems to work—on Earth at least.