From Tom Rees of Epiphenom:

The evolutionary origins of our moral sense is a hot topic at the moment, but what’s becoming clear is that physical and moral disgust are tightly linked (we pull the same faces for both, for example).
Why? Well, moral disgust probably evolved out of the neural systems that originally served to provoke physical disgust. And they’re still linked. Which suggests that the reason we associate cleanliness with godliness is down to a neural short circuit.
In other words, ritual purification may well stem from the fact that we have a cognitive malfunction that makes us associate cleanliness with morality. Assuming that, the interesting question to ask is: What are the consequences? A recent study on clean smells suggests that purification has a morality-reinforcing effect, but there may also be a darker side, according to a study published earlier this year.
This was a study into the effects of hand-washing by Simone Schnall and her colleagues from the University of Plymouth in the United Kingdom. In a cunning experimental design, they quizzed students on morality—but half of them were asked to wash their hands first (they went through some hoops to make sure the students didn’t think the hand-washing was connected to the experiment).
Here’s what they found: Students with clean hands actually rated a series of morally ambiguous actions as less wrong than did students who hadn’t washed their hands.
The difference was particularly big for judgments where the students were asked to imagine themselves doing the action. For example, they were asked to imagine they found a wallet with money in it and the address of the owner, and that they had decided to keep it on the grounds that the owner was rich and they were poor. Is that an immoral act? Well, it’s questionable, of course, but the point is that those who had washed their hands were less likely to think it immoral.
This result reminds me of other studies that suggest people who are very firm in their moral convictions are actually more likely to act immorally and that we seem to have an internal accounting system that adds up good and bad deeds, and pushes us to do bad or good if we’re getting out of equilibrium.
Maybe if you’re in a clean environment, then you act in a morally clean way, but if you personally are ritually pure, then that makes it easier to do morally dubious things.