TORONTO - Buying green may help save the planet, but a new study suggests it might also make you more prone to cheat, steal, and be selfish.

In a paper to be published in an upcoming issue of the journal Psychological Science, University of Toronto researchers Nina Mazar and Chen-Bo Zhong studied how students behaved after being given the option of purchasing environmentally friendly products, like organic yogourt or biodegradable laundry detergent, or conventional items.

They found students who chose green products were less likely to act altruistically afterwards than those who were simply exposed to green products.

The study, said Mazar, an assistant professor of marketing with the University of Toronto's Rotman School of Management, builds on research into the idea of "moral regulation" -- that people either consciously or unconsciously balance bad deeds with good ones.

"What has been shown so far is that when we engage in actions that give us some kind of moral, warm glow -- let's call it that -- that afterwards we are more likely to transgress," Mazar said.

"What we don't know, and what the interesting question is, is how much is really a conscious, deliberate thought process? We don't know that."

In one experiment, students were assigned to one of two computerized "stores" filled with either predominantly green products or conventional items. Once assigned to a store, some students were asked to think critically about the products, while others were told to go shopping.

The students were then given six dollars and told there was a person in another room with whom they were supposed to share the money, keeping whatever they didn't give away for themselves.

The students who were simply exposed to the green items parted with more money than those who were exposed to the conventional products. But when it came to the students who made purchases, the opposite was true: those who bought green items actually gave less than those who spent their money on non-green alternatives.

In a second experiment, students were again asked to purchase either green or conventional items. Then, they were placed in front of a computer and shown a series of images depicting clusters of dots spread across the screen.

If the students told researchers they saw more dots on the left side of the screen, they were paid half a cent. But if they said there were more dots on the right, they got five cents -- regardless of where the dots actually appeared.

After 90 trials, the students learned how much they'd earned and were told to pay themselves by taking their earnings out of an envelope containing five dollars. So not only could they make money by lying to researchers about the number of dots on the computer screen, but they could also increase their payoff by stealing from the envelope.

It turned out students who'd shopped at the eco-friendly store, on average, made 36 cents more on the dot test and stole 48 cents more from the envelope than those who'd made conventional purchases.

While the findings might deflate the self-righteous air of those who brag about bringing canvas bags to the grocery store, Mazar says it definitely shouldn't be seen as a condemnation of environmentally friendly purchasing habits.

That, she feels, would be a gross misunderstanding of the point of the research. The study shows we should be aware of our tendency to treat buying green as a moral act, said Mazar, rather than as our responsibility to the planet.

"What we wanted to point out is if you start to moralize particular actions . . . then there is a danger that people get this kind of warm glow. And that can be used afterwards to engage in less, maybe, social or altruistic behaviour," said Mazar.

"But this doesn't mean that you should not buy environmental products."