There's a classic cartoon plot device that represents a struggle with temptation. A tiny angel pops up on the conflicted character's left shoulder, urging him to follow the path of righteousness. A tiny devil sits on his right shoulder, pressing him to give into his desires.

In real life, it turns out that an everyday item has the power to act as both angel and devil every time we go to the grocery store. It lurks in car trunks and pantries all over the world, waiting to guide us simultaneously down paths of virtue and vice. What is this surprising Svengali?

It's a reusable shopping bag.

“You did something good for the environment, so you can have a cookie.”

New experimental research shows that shoppers are more likely to buy virtuous organic items when they bring their own reusable bags to the store than when they opt for paper or plastic bags at the checkout counter. At the same time, those who bring their own bags are more likely to buy indulgent items like ice cream and cookies. Moreover, consumers tend to place a higher value on both organic products and decadent treats when they bring their own bags than when they don't.

Researchers Uma R. Karmarkar and Bryan Bollinger report their preliminary findings in their working paper BYOB: How Bringing Your Own Shopping Bags Leads to Treating Yourself, and the Environment. (The collaborative effort addresses each of their particular interests. Karmarkar, an assistant professor and neuroscientist in the Marketing unit at Harvard Business School, studies factors that affect consumer choice. Bollinger, an assistant professor at NYU's Stern School of Business, studies the marketing of sustainable products.)

"There are all these little things that we're supposed to do to be better to the environment, like turning off the lights when we leave the room or recycling our bottles," Karmarkar says. "Bringing bags is interesting in that it's a difficult thing to remember to do, and actually requires a fairly big behavioral change on the part of the consumer. Our question was, when you succeed at this big behavioral change, does it change other elements of what you're doing as well?"

A Series Of Experiments

As their working paper explains, the researchers combined empirical and experimental methods to test the purchasing effect of reusable bags.

Looking at loyalty card data from a large grocery chain in California, Karmarkar and Bollinger tracked and analyzed 936,232 purchases by 5,987 households across two years. To assess organic purchases, they looked for transactions in which the consumer could choose either an organic or a nonorganic option—a carton of milk, for example. In monitoring what they called "indulgent" purchases, the researchers looked at sugary items like ice cream and candy bars, as well as salty treats like potato chips.

The data showed a definite correlation: Shoppers who had brought their own bags bought decidedly more indulgences and chose more organic products than those who didn't. But this wasn't necessarily enough information to establish causality—that is, that both effects were specifically due to bringing their own bags. "There are a lot of things going on in a store and a lot of inputs," Karmarkar says.

So she and Bollinger dug deeper with a series of experiments, enlisting participants for a number of online surveys.

In the first experiment, the researchers assigned participants to one of two conditions. The "with bags" participants were asked to imagine approaching a supermarket to do their grocery shopping with their own bags. The "without bags" group received nearly identical instructions, but nothing about bags was mentioned. All the participants looked at a floor map of the grocery store and listed 10 items they would most likely purchase on their hypothetical outing.

Regarding indulgent items, the results depended on whether the participants had children in their households. For those with dependents, there was no significant difference between the with-bags and the without-bags condition. For those without children, the with-bags participants were more likely to imagine buying ice cream and potato chips than the -without-bags- participants.

But the results couldn't speak to organic items; while participants listed items such as milk and vegetables, they generally didn't list whether their hypothetical choices were organic milk and vegetables.

"We could support some of the story but not all of it yet," Karmarkar says.

And so she and Bollinger conducted a second experiment, in which participants reported how much they'd be willing to pay for each of nine specific products. These included both organic and indulgent items, as well as "baseline" items like canned soup. Again, the participants were divided into hypothetical conditions of "with bags" and "without bags."

Consistent with the empirical data, the idea of bringing their own bags increased the likelihood that participants would buy both indulgent and organic items. Moreover, it increased the amount of money they'd be willing to pay for those items.

But the researchers had another question: Does it matter whether a reusable bag is the consumer's choice? "We wanted to examine whether it was important that you made the decision to bring the bags as opposed to a store policy that requires it," says Karmarkar, noting that some stores obligate customers to bring their own bags; others charge customers a fee for single-use carryout bags per a local government mandate.

In the next experiment, all the participants imagined bringing their own bags to the hypothetical grocery store. But while some were told to imagine bringing reusable bags of their own volition, others imagined that they had to bring bags due to a store policy.

Participants then rated their willingness to purchase organic, indulgent, and baseline items. In this case, the results showed no significant difference between the two groups with regard to organic items, which rated highly across the board. However, participants were more likely to buy indulgent foods if they imagined that bringing bags was their own choice.

"A simple way I think about those results is that if you do something good, you reward yourself," Karmarkar says. "You did something good for the environment, so you can have a cookie."

Implications

For retailers, the results suggest that store managers should reconsider where they display their organic items. In short, it may make sense to locate the kale near the Kit Kats.

"The research implies that the area near the checkout counter is a good place to display organic or environmentally positive items," Karmarkar says. "That's the place where shoppers' attention is probably going to be most focused on this element, the bag, which seems to encourage them to buy these things."

For consumers, she recommends that they just think about the findings as they stroll down the grocery store aisles.

"I'm of the mindset that it's useful to know the kinds of things that influence your own behavior," Karmarkar says. "If you're trying to maintain a strict diet, maybe you can recognize the bag's influence, and consciously fill the desire to treat yourself in another way that doesn't interfere with your goals. Maybe you can treat yourself to an extra half hour of sleep."