CAMBRIDGE, Mass. (CBS Connecticut) — A new study finds that people who bring reusable bags to grocery stores end up buying more junk food.

Uma Karmarkar of Harvard University and Bryan Bollinger of Duke University found that people who bring their own bags to stores are more likely to purchase organic food, as well as junk food.

“Grocery store shoppers who bring their own bags are more likely to purchase organic produce and other healthy food. But those same shoppers often feel virtuous, because they are acting in an environmentally responsible way. That feeling easily persuades them that, because they are being good to the environment, they should treat themselves to cookies or potato chips or some other product with lots of fat, salt, or sugar,” the authors write in the study.

Karmarkar told the Harvard Business Review that consumers feel they’ve “earned it” when behaving well in a situation.

“In consumer psychology the word ‘licensing’ is the key. If I behave well in one situation, I give myself license to misbehave in another, unrelated situation,” she said. “Similar research has also been done on health decisions. I get a Diet Coke; I treat myself to a hamburger. In this case bringing a bag makes you think you’re environmentally friendly, so you get some ice cream. You feel you’ve earned it.”

Karmarkar and Bollinger conducted the study by collecting loyalty cardholder data from a single location of a major grocery chain in California between May 2005 and March 2007, comparing the same shoppers on trips for which they brought their reusable bags to trips for when they did not.

“Among those who sometimes brought them you could see different behavior based on whether or not they had their bags with them. It’s great data,” Karmarkar told the Harvard Business Review.

Karmarkar and Bollinger also recruited study participants online and assigned them one of two situations: bringing or not bringing their own bags. The participants were presented with a scenario and a floor plan of the store and were asked to list ten items they were most likely to buy.

“In the experiments, when we told people, ‘Imagine you’re going shopping. You put your own bags in the cart because the store requires you to bring them,’ the junk food purchases stopped, though the organic purchases did not,” Karmarkar told Harvard Business Review. “Basically, if it’s compulsory, we don’t get that jolt of affirmation about being a good person. Just like with recycling. Interestingly, we focused on consumers in the United States, where bringing your own bags is a relatively new phenomenon. I suspect that in parts of the world where this is the norm, you wouldn’t see the effect.”

Karmarkar added, “Another situation where the effect seemed to vanish is when people had kids. With people who bought baby items, the junk food effect disappeared. If you think about it, those shoppers have many more competing motivations, such as being role models and keeping their kids healthy.”

The researchers concluded that their findings can have important implications for grocery store managers to better market organic or sustainably farmed foods.

The study is forthcoming in the Journal of Marketing.