Are you filling your reusable shopping bag with junk food?

Grocery shoppers often feel virtuous when they bring their own reusable bags, given that they're taking an earth-friendly step.

But what happens when the shoppers hit the stores? Do they compensate for that halo of environmental goodness in another -- sometimes not entirely positive -- way? The answer, according to Uma R. Karmarkar of Harvard University and Bryan Bollinger of Duke University, is that one healthy move apparently is often accompanied with a less than virtuous purchasing decision: buying junk food.

First, the good news: When shoppers bring their own bags, they're more likely to buy organic foods, a gesture that jibes with the ethos of bring-your-own-bag: Organic food is often regarded as more earth-friendly than conventional products. But those shoppers are also more likely to toss in some junk food on top of their organic carrots, the study found.

"Bringing one's own bags increases purchases of not only environmentally friendly organic foods but also indulgent foods," Karmarkar and Bollinger wrote in the paper. Because bringing reusable bags feels virtuous, "consumers could feel licensed to make other less virtuous or more indulgent decisions afterward," they noted.

With stores increasingly promoting the use of reusable bags by selling inexpensive cloth or recycled plastic bags, the authors noted that they wanted to research how the bags might influence shoppers' behaviors. It's not a small issue, considering that the research paper noted about one-third of consumers said they used their own reusable bags on their most recent shopping trip.

So while it's becoming mainstream, the adoption of reusable bags still has a way to go, with implications for grocery stores and other retailers.

Why do people who make the effort to tote their own bags break down when they get to the junk-food aisle? The reason is a concept in consumer psychology called "licensing," Karmarkar told the Harvard Business Review.

"If I behave well in one situation, I give myself license to misbehave in another, unrelated situation," she said. "Similar research has also been done on health decisions. I get a Diet Coke; I treat myself to a hamburger."

In the case of reusable bags triggering a junk-food purchase, the shopper may not be conscious of the behavior, Karmarkar cautioned.

"Bringing bags to the store isn't related to pulling products off the shelf, and yet it has an impact," she noted.

The study analyzed loyalty card data from a single location of a major California grocery chain from May 2005 to March 2007, and compared the same shoppers on trips when they brought their own bags and when they used store bags.

More than 2 million shopping trips from almost 60,000 households were in the data, although the researchers focused on the most frequent shoppers within that group. The study also included an online component, where the researchers asked people through Amazon's Mechanical Turk questions about hypothetical shopping decisions.

What impact could this finding have on stores? Grocery stores where shoppers often bring reusable bags might want to market organic or sustainably farmed foods as indulgences, the authors noted in a statement.

Stores could also tweak how shoppers experience their check-out, Karmarkar told the Harvard Business Review.

"What might happen to impulse buys at checkout if you reminded shoppers that they're great for bringing in reusable bags?" she said. "Does the placement of the indulgence in the store matter? The flip side is that you could see what happened if you offered organic cookies! Something that is both virtuous and indulgent."

On the other hand, pointing out the behavior might help shoppers who are trying to cut back on junk food. The next time you enter a store with a reusable bag, you might want to steer clear of the snack aisles.