The double-edged sword of… reusable shopping bags?

Using a reusable bag to shop for groceries may be good for the environment, but it won’t be doing wonders for your own waistline, a new study suggests.

The report from Harvard Business School, which looked at the buying habits of thousands of shoppers in California, revealed that those who bring a reusable bag to the grocery store are more likely to buy organic foods — but are also more prone to splash out on junk food.

“We find that when people bring their own grocery bags, they are more likely to buy organic produce, and they are also more likely to buy indulgent items like cookies, ice cream and snack chips,” Uma Karmarkar, assistant professor of marketing at Harvard and co-author of the report, tells us.

Karmarkar and his co-author, Duke University’s Bryan Bollinger, analyzed about one million transactions at a grocery chain over a two-year period, using data from customer loyalty cards.

How did they know the junk food splurges were because we’re congratulating ourselves on being eco-minded shoppers? When the grocery store required shoppers to bring their own bags, they didn’t buy the unhealthy foods.

“People feel they have done something good or virtuous,” Karmarkar explains. “This creates what is called a licensing effect. Having already done one virtuous thing, people feel licensed to indulge more in later choices.”

So next time you’re shopping, think of the sea creature you could be saving by not using plastic bags rather than grabbing a bag of sea salt chips as a treat for yourself.