Follow CBSDFW.COM: Facebook | Twitter

NORTH TEXAS (CBS 11 NEWS) – Heather McNamara of Dallas has used reusable grocery bags for as long as she can remember. And more so after Dallas passed a plastic bag ban. “I would come home with 15 to 20 grocery bags,” she told CBS 11 News. “It is wasteful, obviously.”

While people might be saving the earth, researchers found that by using reusable bags, some allow themselves to put more junk food in their shopping carts. The idea surprised McNamara. “It doesn’t make a lot of sense, but it might be true,” she said.

Harvard and Duke University researchers studied thousands of grocery receipts from shoppers. They found that those who use reusable totes buy a lot more organic foods but, they also end up filling their carts with more junk food than an average plastic bag user.

Experts call it “Moral Licensing.” Dr. Ritesh Saini, Marketing professor at the University of Texas at Arlington said it happens subconsciously. “One good thing liberates you, in the sense, it gives you a license to engage in something that’s not all that great.”

Moral licensing is not limited to reusable bags alone. It’s a phenomena many of us experience in daily life… such as eating a piece of cake after a big workout.

“Seems like we are keeping some kind of a mental clock ticking in our mind,” Dr. Saini explains. “We are giving ourselves positive or negative brownie points.” And since it is our sub-conscience at work, we don’t even realize when we do that.

To avoid getting into the tote bag trap – it comes down to will power and perhaps awareness. Which McNamara has a lot of now. “Definitely, when I go to the grocery store, I will be more aware of what I’m putting in my basket.”

Another surprising finding of the study could help those Dallasites stuck with a plastic bag ban. Researchers found that people in cities where they are required by law to use reusable grocery bags bought less of the sugary treats.

(©2015 CBS Local Media, a division of CBS Radio Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.)