New research suggests that reusable shopping bags not only signify that you're environmentally friendly; it also suggests that you frequent the produce and junk food aisles.

Researchers from Harvard and Duke Universities found that reusable bags influences the very things grocery shoppers buy, making them more likely to purchase organic and junk food.

"Grocery store shoppers who bring their own bags are more likely to purchase organic produce and other healthy food. But those same shoppers often feel virtuous, because they are acting in an environmentally responsible way. That feeling easily persuades them that, because they are being good to the environment, they should treat themselves to cookies or potato chips or some other product with lots of fat, salt, or sugar," write the authors, Uma R. Karmarkar of Harvard University and Bryan Bollinger of Duke University.

For the study, researchers collected and analyzed loyalty cardholder data from a single location of a major grocery chain in California between May 2005 and March 2007. They compared the same shoppers on trips for which they brought their own bags with trips for which they did not. Participants were also recruited online from a national pool and were randomly assigned one of two situations: bringing their own bags or not bringing their own bags. Depending on the situation, participants were presented with a certain scenario and a floorplan of the grocery store and were asked to list the ten items they were most likely to purchase on the trip.

They found that when shoppers brought their own reusable bags, they were more likely to purchase organic foods. At the same time, they were also more likely to purchase junk food. And both results were slightly less likely when the shopper had young children: parents have to balance their own purchasing preferences with competing motivations arising from their role as parent.

"In short, bringing your own bags changes the way you shop," write the authors. "Our findings thus have important implications for grocery store managers. In stores where reusable bags are popular, marketing organic or sustainably farmed foods as indulgences could increase the sales of those items."

The findings are published in the Journal of Marketing.