It's a commonplace that people sometimes do good deeds to expiate their sins. What's not quite so well appreciated is that the opposite also happens; doing a good deed gives us licence to behave badly. A new paper establishes this experimentally. Researchers got people to play a sequence of simple dictator games. And they found that the dicators' decisions on how much to give away was not constant but rather cyclical; high donations followed low ones and low ones followed high. People who behaved altruistically once behaved selfishly the next time:

Donations over time seem to be the result of a regular pattern of self-regulation: moral licensing (being selfish after altruist) and cleansing (altruistic after selfish).

Moral behaviour, then, far from being a stable effect of our character, seems to be cyclical. The word "error" appears in "fundamental attribution error" for a reason.

This is not always a bad thing. As Anna Merritt and colleagues say (pdf), moral licensing can give people the wiggle room to make necessary but morally unpleasant choices - for example, a doctor having to choose which ill patient to treat.

But here's my concern. Robert Cialdini has shown how, sometimes, people over-reciprocate. A small initial favour, he says, can yield large favours in response. Could a similar thing be true for moral licensing, whereby we feel that smallish good deeds give us licence to do large bad ones?

Two things make me fear so. One is simply that humans have great capacity for self-justification: "they won't miss the money"; "she's up for it really" and so on. Big misdeeds thus shrink quite easily.

The other is yet more experimental evidence.Tobias Regner and Gerhard Riener got people to play a trust game, in which two trustees were given the choice to repay the trust a third person had put in them. They asked: how does a trustee act, if he sees his fellow trustee's behaviour? They found that when the first trustee did reciprocate, 36 of 43 followers did not. And when the first trustee didn't reciprocate, 14 of 15 followers didn't. In other words, whatever the first trustee does, the second overwhelmingly behaves selfishly.

What's going on here, say Regner and Riener, is moral cherry-picking. People choose the motive that promotes their self-interest.So if the leader reciprocates, the follower thinks "the obligation to the trustor has been repaid already, so I don't need to do so." And if the leasder doesn't reciprocate, the follower thinks "Hey, there's nothing wrong with not reciprocating - I'm only doing the same as the other fellow."

You might - if you wish - see an application of this to current scandals.But that's not really my point. Instead, I'm agreeing with Norm; you don't have to be a conservative to fear that our moral conduct is fragile, and we have impulses that need restraining.