CAMBRIDGE, Mass. – New research from Harvard Business School shows that people who bring reusable bags to the supermarket are also more likely to splurge on items like ice cream and snack food.

Researchers found, through studying the household bills of thousands of shoppers in California (which recently approved a ban of plastic bags altogether), that while people who bring their own bags to supermarkets are more likely to buy organic foods, perhaps surprisingly, they are also more likely to buy junk food.

Uma Karmarkar, an assistant professor of marketing at Harvard Business School, and her research partner Bryan Bollinger, of Duke’s Fuqua School of Business, studied about one million transactions at a California shopping chain over two years, using data from customer loyalty cards.

They found that people with reusable bags were 0.25% more likely to buy organic versions of staple foods, such as milk and eggs. But they were also more likely — by 1.24% — to buy treats such as chips, ice cream and cookies.

In a post on the Harvard Business School’s website, Professor Karmarkar said that shoppers were probably justifying buying the sugary, high-calorie snacks because they had done a good deed by bringing their own bags.

“In consumer psychology the word 'licensing' is the key,” she explained on the site. “If I behave well in one situation, I give myself license to misbehave in another, unrelated situation. 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.”

The researchers did admit, however, that the patterns they detected might have been different in another location, but said the survey still opened up interesting questions about shoppers' habits and how to influence them.