With apologies to Kermit the frog, turns out it can also be sleazy being green.

People who buy goods perceived to be morally good are more likely to go on to lie, cheat and steal, according to a study by researchers at University of Toronto's Rotman School of Management.

Nina Mazar and Chen-Bo Zhong, inspired by a shared interest in green living and an appreciation of how the human mind can justify bad behaviour, found that people who are simply exposed to green products behave more altruistically than people who purchase the products.

Before any green-bashing begins, the authors stress that the point of the study was not to finger green consumers, but to demonstrate that buying anything seen as morally good may make you feel better but may also give you a licence to behave badly – and, as this study found, in ways completely unrelated to the "good" behaviour.

"It's almost like a zero-sum game going on, and it's unclear how much of it are we really aware of," says Mazar, an assistant marketing professor. "It seems to be so ingrained into our human nature."

The study, titled Do Green Products Make Us Better People? and to be published in an upcoming issue of Psychological Science, used U of T students as guinea pigs, and the researchers conducted three separate experiments to make their point.

In experiment number one, a survey determined that the students believe people who buy green products to be more cooperative, altruistic and ethical than people who bought non-green products.

In step two, an online browsing and shopping experiment involving two stores stacked with either green or conventional products, combined with a seemingly unrelated "dictator" game that allowed each player to choose how much of $6 provided he or she would share with an unseen person in a room next door, produced expected yet counterintuitive results.

Those who had browsed the store that contained more green products but had made no purchases shared more money than those who had browsed the store with conventional products, but this result flipped for those who had bought from the green store. They shared less money than those who bought from the conventional store.

In the third experiment – and this is where the lying, cheating and stealing comes in – students were seated at computer terminals with envelopes containing $5 in loose change. They were then randomly assigned to make purchases in either the conventional or the green store.

Then, they were given a seemingly unrelated computer perception task where they would see, for just a second, 20 dots scattered inside a box, divided by a diagonal line. They were then told to pick the side that had more dots.

Students were told they'd be paid 5 cents for each trial in which they identified more dots on the right side of the dividing line, and only a half a cent for those that they identified as having more dots on the left. During a monitored "test" trial, students discovered the game paid out according to the side that had been chosen, regardless of whether they had chosen correctly or not.

The money round had 90 trials. At the end of the run, the amount earned was calculated on the computer screen and they were then told to take that amount from the envelope in front of them.

Those who had bought from the conventional store lied and cheated significantly less than the green store shoppers, who were more likely to pick the high-paying side, regardless of where the dots were.

Finally, the researchers counted the change left in the envelopes and found the green buyers outdid the conventional buyers at pocketing more than the computer calculated they had earned. On average, the green buyers left with an extra 83 cents in their pockets.

Mazar isn't sure how this new study, which demonstrates that buying something good can make you bad, will apply to her chosen field of marketing, but says it raises bigger questions about human behaviour and how to undo it, which may prove quite tricky, since this is all quite human.

"It's a very fine line that can change the behaviour in the opposite direction," she says. "It's actually quite a challenge. How can we help people to actually behave in line with their moral values, and to not fall into this licensing mode that can happen?"

Has Mazar ever caught herself engaging in the sort of behaviour she studies?

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No, she says, but then wonders how one could be sure of such a thing.

"I want to think of myself as a moral human being and with high moral values. If I were to engage in something that is a little bit less moral, I'm sure I would find all kinds of justifications that make it so that it doesn't feel so bad," she says. "Humans have a lot of tricks to deal with such situations when they are tempted to be dishonest or selfish.

"It makes us very interesting," she says. "It's nothing bad. It's just how it is."