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Around the country, a movement is underway to stop grocery shoppers’ reliance on plastic bags. People are being encouraged, sometimes financially, to bring their own reusable bags. But one recent study has shown that bringing reusable bags to the grocery store can also affect the way people shop.

The research, published in Harvard Business Review, had some results you might expect. People who used reusable bags tended to buy more organic foods, with one green action leading to another. But one unexpected trend jumped out from the data: People who brought their own bags bought more junk food as well, like ice cream, chips, candy and cookies.

Uma Karmarkar, an assistant professor of marketing at Harvard Business School, teamed with Bryan Bollinger of Duke’s Fuqua School of Business. They looked at California shopping receipts that indicated which shoppers brought their own bags and which did not. They were also able to look at individual households and compare trips to the store when they brought bags and when they didn’t. “We looked over a long period of time, and you could spot the people who always brought bags and the people who sometimes did,” Karmarkar said. “Among those who sometimes brought them, you could see different behavior based on whether or not they had their bags with them.”

One of those different trends, as mentioned above, was an increase in junk food purchases. Though the data only provides a correlation, researchers speculated on possible reasons for the findings. “You give yourself a cookie. In this case, literally,” Karmarkar believes. “If I behave well in one situation, I give myself license to misbehave in another, unrelated situation.” Other research has supported similar phenomena. And when participants were given similar, theoretical shopping scenarios—one when they were told to think about shopping but didn’t actually buy anything—they didn’t make the same choices, meaning the behavior may be subconscious.

More research needs to be done, but the moral: Being green may not be good for your waistline. Though saving an entire planet is probably more important than fitting into your swimsuit.