Bringing reusable bags to the grocery store brings self-identification as an environmentally friendly shopper, but it also influences the things you buy, according to a new paper in the Journal of Marketing.

Reusable bags were correlated to organic food - no surprise there - but also 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 authors Uma R. Karmarkar of Harvard University and Bryan Bollinger of Duke.

The study is one of the first to demonstrate that bringing one's own grocery bags causes significant changes in food purchasing behavior. The authors collected 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.

The researchers found that when shoppers brought their own bags, they were more likely to purchase organic foods. At the same time, bringing one's own bags also increased the likelihood that the shopper would 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 parents.

"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."

Citation: Uma R. Karmarkar and Bryan Bollinger, "BYOB: How Bringing Your Own Shopping Bags Leads to Treating Yourself and the Environment", upcoming in the Journal of Marketing.