With consumers using nearly 1 trillion plastic bags each year worldwide, reusable grocery bags provide an environmentally conscious alternative. While these bags help to reduce our impact on the environment, they might also be influencing the way we buy food.

In a study in the Journal of Marketing, researchers from Duke University and Harvard University observed shoppers in California from May 2005 to March 2007 who both brought their own bags and used plastic bags.

They found that shoppers who brought their own bags were more likely to buy organic produce than the shoppers who used plastic bags. Another interesting finding, however, was that shoppers who brought their own bags also purchased more junk food than the plastic bag users.

The reasoning behind this change, the researchers said, was that grocery shoppers who brought their own bags felt “virtuous” about helping the environment, so they felt the need to treat themselves with products with lots of fat, salt or sugar. The researchers say this phenomenon is called “licensing.”

On top of buying unhealthy foods, the researchers found reusable bag shoppers ended up spending around $12 more per week on groceries than plastic bag users.

The study’s researchers believe that this insight could have implications for store managers who want to increase sales. In grocery stores where reusable bags are popular, the researchers suggest that managers should market organic foods as indulgences so that the reusable bag shoppers would want to purchase those items.

The next time you’re at the grocery store with your reusable bags, keep this study in mind when you reach for junk food. Perhaps consider a fancy fruit to be your treat.