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The research: Uma Karmarkar, an assistant professor of marketing at Harvard Business School, and her research partner Bryan Bollinger, of Duke’s Fuqua School of Business, studied the grocery bills of thousands of California shoppers. Because the receipts noted small discounts given to people who had provided their own reusable bags, it was possible to study whether their purchases differed from those of other shoppers. Comparisons showed that people who brought their own bags were more likely to buy organic goods—but were also more likely to load up on high-fat, high-calorie junk.

The challenge: Do we tend to indulge ourselves with treats when we feel that we’ve done the right thing?

HBR: Professor Karmarkar, defend your research.

Karmarkar: It was clear that shoppers who brought their own bags were more likely to replace nonorganic versions of goods like milk with organic versions. So one green action led to another. But those same people were also more likely to buy foods like ice cream, chips, candy bars, and cookies. They weren’t replacing other items with junk food, as they did with organic food. They were just adding it to their carts.

There are so many variables in a grocery store. How did you isolate this behavior?

Bryan did amazing quantitative analysis on this. The data we worked with was based on loyalty cards, so we didn’t just compare individual transactions. We could compare the purchases of the same people when they brought bags versus when they didn’t. And we could eliminate shopping trips that didn’t look like your canonical weekly trip to the grocery store. So we got rid of outlier transactions, like ones in which it was clearly a small business doing the shopping or someone picked up only two items.

So it’s not just that the kind of people who bring bags are more likely to buy organic?

It doesn’t appear to be. We looked over a long period of time, and you could spot the people who always brought bags and the people who sometimes did. Among those who sometimes brought them you could see different behavior based on whether or not they had their bags with them. It’s great data.

So this is the classic indulgence: You do good and—

You give yourself a cookie. In this case literally. In consumer psychology the word “licensing” is the key. If I behave well in one situation, I give myself license to misbehave in another, unrelated situation. Similar research has also been done on health decisions. I get a Diet Coke; I treat myself to a hamburger. In this case bringing a bag makes you think you’re environmentally friendly, so you get some ice cream. You feel you’ve earned it.

But the two acts aren’t really connected. Why would something I take out of the car and drop in the bottom of the cart affect my choices after that?

We have some evidence that it’s the awareness of taking the action that triggers the behavior. It could be subconscious. I don’t think people are actively thinking, “I’m using reusable bags, so I will get some doughnuts.” This is one of the reasons it’s interesting to me in the first place. Bringing bags to the store isn’t related to pulling products off the shelf, and yet it has an impact.

But I suspect that as bringing reusable bags becomes a widespread practice, it’s likely these effects will change. Look at bottle recycling. It used to be that you felt as if you were doing a good thing by recycling bottles. Now it’s to the point where you don’t get a cookie for recycling them; you just get penalized if you don’t. You get nasty stares.

Were there any cases in which people who brought their own bags didn’t do the organic food–junk food thing?

In the experiments, when we told people, “Imagine you’re going shopping. You put your own bags in the cart because the store requires you to bring them,” the junk food purchases stopped, though the organic purchases did not. Basically, if it’s compulsory, we don’t get that jolt of affirmation about being a good person. Just like with recycling. Interestingly, we focused on consumers in the United States, where bringing your own bags is a relatively new phenomenon. I suspect that in parts of the world where this is the norm, you wouldn’t see the effect.

Another situation where the effect seemed to vanish is when people had kids. With people who bought baby items, the junk food effect disappeared. If you think about it, those shoppers have many more competing motivations. Such as being role models and keeping their kids healthy.

Do you have any sense of the value of the indulgence? How much do we spend when we feel so eco-awesome?

This is where indulgence gets tricky. The dollar value of the indulgence relative to the entire basket’s value tends to be low. But the nature of the food—high calorie, high fat—may be the more important factor, not how much it costs. The effect does dissipate as indulgences get more expensive. Then there’s a whole other, nonfood aspect to it. Is lavender-scented laundry soap an indulgence? Maybe. We limited our focus to food.

What else can be studied with regard to grocery indulgences?

Location may matter. The effect may be different in Boston than in California. One question I’m interested in is, What might happen to impulse buys at checkout if you reminded shoppers that they’re great for bringing in reusable bags? Does the placement of the indulgence in the store matter? The flip side is that you could see what happened if you offered organic cookies! Something that is both virtuous and indulgent. And how could these effects be used to help shoppers make choices that increased their happiness? There’s so much you could do.

It sounds as if you could spend a whole research career just studying grocery stores.

[Nervous laughter] Maybe, but I don’t think I’ll do that.

This was a really good interview. I feel that I’ve done a good job on it. I’m going to take the afternoon off.

That would be a typical indulgence! I think it’s fun that humans are like this, that these subtle subconscious effects can move our decisions around. It’s something that makes us interesting.