Kermit the Frog may have got it only half right when he said "it ain't easy being green." A new study suggests that when you're green, it ain't easy being nice, either.

The study, conducted by two University of Toronto professors, found that consumers who bought environmentally friendly products were less likely to be altruistic and more likely to cheat and steal.

"Purchasing green products may produce the counterintuitive effect of licensing asocial and unethical behaviours by establishing moral credentials," the study found. "Thus, green products do not necessarily make us better people."

Titled Do Green Products Make Us Better People?, the study by Nina Mazar and Chen-Bo Zhong is set to be published in the next issue of the Psychological Science journal.

Mazar, a self-described green consumer, says she was surprised that buying green products appeared to sanction bad behaviours. "It went beyond just how nice people were to others," she told CBC News.

The professors found that participants who bought green products in the study were also more likely to lie and steal money.

Think twice, author urges

In one part of the experiment, 90 University of Toronto students were seated in front of a computer and asked to identify whether the screen had more dots on the left or right, knowing they would receive a nickel for one side and half a cent for the other.

Volunteers who had bought green products not only lied to earn more money, but stole additional money out of an envelope with $5 worth in change, which was set beside the computer for participants to take their earnings from.

Merely exposing participants to green products, however, seemed to trigger more altruism. In an anonymous game played by 156 students, those shown an online store with mostly environmentally friendly products gave away more money to a partner than those who browsed a store with few green goods.

But the results were flipped if the participants bought products. Then, those who bought products in the green store shared less money than those in the regular store.

So while exposure seemed to increase "subsequent pro-social behaviour," acting on it seemed to give licence for "deviating behaviour," the study says.

As for Mazar, she can't personally recall an incident where she felt like a green purchase led her to a later misdeed. "At the end of the day, it's not so clear how conscious these kinds of licensing effects are," she says.

She hopes the study makes others think twice about their conduct. "It is important that they don't feel morally superior just because they have recycled something."