The researchers arrived at their conclusions after putting 90 undergraduates through a simulated online shopping trip. Some shoppers bought green products while others bought conventional products. The shoppers were then paid to play an online counting game that gave them an easy opportunity to cheat. The researchers found that undergraduates who had shopped at the green stores were significantly more likely to be dishonest.

And, turns out, people who believe that being green gives them a free pass to behave badly in other areas aren't likely to have second thoughts about it, the researchers say. "Our paper shows its very hard to change human behavior in the long-term. Morality, to some degree, may be a zero-sum game," says Mazar, referring to the study published in an upcoming issue of the journal Psychological Science.

These apparent contradictions abound. Research published in 2009 showed that people who were praised for being humanitarians were less likely to make charitable donations. Other research has showed similar licensing effects in situations involving gender prejudice, among other areas.

The green-shopping study provides the latest evidence of how difficult it might actually be to change human behaviors -- and moral equilibrium -- over any extended period. Equally important, psychologists believe this license to be bad is not necessarily connected to any one area.

Picking up a piece of litter on the street, for example, may not mean that someone feels comfortable throwing away a plastic bag the next day. Rather, the Good Samaritan who picks up trash may feel less compunction in lifting a sampling candy at the bulk foods section of the Whole Foods (WFMI) store.

In the latest experiment, the shopping and counting game portions of the experiment were completely unrelated. Some 90 students from the University of Toronto volunteered to participate for a payment of 5 Canadian dollars ($4.84). The researchers randomly assigned participants either to an online store with green products, or one with conventional products.

On the desk with the computers used in the experiment was an envelope containing CA$5 in varying denominations. After a simulated shopping spree, the participants ran through several rounds of the counting game. Mazar and Zhong found that those who had purchased green products were far more likely to cheat on the counting game in a way that allowed them to win more money for their counting performance.

To hammer home the point, Mazar and Zhong let the participants take money out of the envelope on an honor-system basis to pay for their counting game performance. Shoppers at the eco-friendly store doubled down and took more money out of the honor-system envelope than they were supposed to -- far more than the participants who bought conventional products.

Does shopping for green products make us feel OK about being dishonest? It's true that the subjects were observed in a lab, not in actual stores. But the results were strong and the findings corroborate previous research on the topic.

For her part, Mazar hopes that future research will yield ways to minimize the so-called licensing effect and quiet the constant counting of good deeds versus bad deeds deep inside the human mind.

What's the moral of this story? Be wary of your next-door neighbors who go on about how green they are and show off their shopping bags -- reusable of course -- filled with green goods. They may be the ones cheating at poker and swiping the newspaper off your apartment stoop.

Alex Salkever is Senior Writer at AOL Daily Finance covering technology and greentech. Follow him on twitter @alexsalkever, read his articles, or email him at alex@dailyfinance.com.