Reusable shopping bags have interesting effects on consumers. Enter a grocery store armed with your own bags and you are, according to new research, more likely than the average shopper to do two things -- buy organic produce and junk food.

The conclusions of the new study came after researchers Uma R. Karmarkar of Harvard University and Bryan Bollinger of Duke University analyzed the purchasing habits of loyalty cardholders at a California grocery store over the span of two years -- between May 2005 and March 2007.

The findings were confirmed by an online simulation and survey, whereby participants were asked -- after being presented with various shopping scenarios and grocery store floor plans -- what products they'd be most likely to buy.

"Grocery store shoppers who bring their own bags are more likely to purchase organic produce and other healthy food," scientists wrote in their paper, published this week in the Journal of Marketing.

"But those same shoppers often feel virtuous, because they are acting in an environmentally responsible way," the researchers added. "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."

Both tendencies were less pronounced when shoppers were accompanied by young children.

"In short, bringing your own bags changes the way you shop," the study's authors concluded. "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."