By Chris McKinnon, WBZ-TVBy Chris McKinnon

BOSTON (CBS) — Reusable shopping bags are considered one of the easiest ways to be environmentally conscious. One shopper at the Market Basket in Waltham told us that is why she uses these bags.

Indeed, Harvard Business School professor Uma Karmarkar found our shopping habits change when we use these bags. Surprisingly however, it has nothing to do with being earth friendly.

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“On the trips where people brought their bags, they seemed more likely to buy organic,” explained Karmarkar.

While that might make some sense, Karmarkar’s other finding was a bit unusual.

“They were also more likely to put an indulgence in their basket, and we defined indulgence as cookies, ice cream, and potato chips,” said Karmarkar.

Karmarkar believes this ties into previous research about behavior known as the “Licensing Effect”.

“When you do something good in one domain, you potentially, unconsciously are more likely to indulge in another one,” she said.

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Shoppers we spoke to told us they were not surprised to hear that people do this internal negotiation.

“If you are doing a good thing then maybe you should take care of yourself as well . . . treat yourself to something,” said one shopper.

Another woman agreed. “I think it makes perfect sense. We always want to reward ourselves when we do something that we think is extra.”

Still, Karmarkar was a little surprised by this application of this theory. Usually a reward is directly linked to an action, such as eating a dessert because you exercised earlier in the day.

There is no real link between environmentally conscious behavior and these high calorie rewards.

One possible conclusion is people end up feeling pretty virtuous about saving the planet.

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“You do something good for society in the morning, and you might be a little more likely to have an extra donut in the afternoon,” added Karmarkar.