Just when you thought it was safe to head to the store with your reusable totes in tow, a new study finds that your reusable bags may not be as cool as you thought. In fact, they may even be making you fat.

According to researchers at Harvard Business School and Duke University, a shopper's behavior is influenced by the type of bag she uses at the store. Shoppers who use reusable bags tend to buy more healthy and environmentally-friendly items — as well as more junk food and other unhealthy splurges.

Why would shoppers who are obviously doing their part to help the environment be more likely to succumb to purchases that are bad for both their health and the planet? The theory is that using a reusable bag at the store makes you feel more virtuous. Shoppers who use these bags feel like they deserve a pat on the back for being such good environmental stewards, and that they should reward themselves — maybe with a doughnut or a bag of chips.

"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," said the researchers in a paper published in the Journal of Marketing. "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."

For the study, researchers recruited volunteers who were randomly assigned one of two situations: bringing their own bags or not bringing their own bags to a store in which they were given the floor plan and told to list the 10 items they were most likely to buy. This is interesting, but also hypothetical. The things people say they will do and the things they really do often don't match up.

But the real evidence on shopping patterns came when researchers pored over loyalty card data collected from a major grocery chain in California collected between May 2005 and March 2007. From this information, they were able to determine the things the same shoppers purchased when they brought reusable bags and the things they purchased when they didn't.

Not surprisingly, researchers found that when participants brought their own shopping bags to the store, they also bought more organic and Earth-friendly items. But they also tended to buy more junk food. The effect was greatest for shoppers without children when compared to those who did have kids. Probably because parents may have to balance their desire for a reward with their responsibility to set a good example for their kids.

It's good to keep in mind that whenever you are shopping — whether you have your own bags or not — it's better for your wallet, the environment and your waistline if you bring a list to the store and stick to it. And if you are going to pat yourself on the back with a treat, at least go for something that's as good for the planet as those bags your bringing along — like organic chocolate.

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