If we think we’re already pretty good, the logic goes, why should we waste resources trying to be good — we’ve already accomplished goodness.

A new study (abstract with link to free PDF), set for publication in Psychological Science, takes this idea a step further, and in a way particularly relevant to our suddenly “green” product rich world. What is the effect of all these green buying options suddenly available to millions of Americans. Well, in short:

In line with the halo associated with green consumerism, people act more altruistically after mere exposure to green than conventional products. However, people act less altruistically and are more likely to cheat and steal after purchasing green products as opposed to conventional products.

That is, exposure to the concept of “greenness” makes us act better; but indulgence in the actual purchasing of green products makes us act worse.

An article in Slate describes the experiment:

In an experiment, participants were randomly assigned to select items they wanted to buy in one of two online stores. One store sold predominantly green products, the other mostly conventional items. Then, in a supposedly unrelated game, all of the participants were allocated $6, to share as they saw fit with an anonymous (and unbeknownst to them, imaginary) recipient. Subjects who had chosen items from the green store coughed up less money, on average, than their counterparts. In a second experiment, participants were again assigned to shop in either a green or conventional store. Then they performed a computer task that involved earning small sums of cash. The setup offered the opportunity to cheat and steal with impunity. The eco-shoppers were more likely to do both.

The environment is certainly not the only place where this idea of moral self-regulation pops up. Another recent study found that white people who voted for Obama later appeared to feel they had more license to be racist in other aspects of their lives.

Alternately, research has shown the effect to work in the reverse. When people are made to feel negatively about their moral status, they’ll act better to try to compensate.

The ultimate lesson, I think, is that our motives are rarely what we think they are. We think we want to do good to do good, but more likely we want to do good because we feel guilty. Likewise, those of us who think we’re good people, we’re probably the ones who act the worst — because we think we’ve got no moral deficit to pay off.

So, buy green this holiday season. That green product is a green light to act like a real bastard the rest of the year.