We like to reward ourselves for good behavior. We go for a walk outside and reward ourselves with a soda. Even when we bring our own reusable grocery bags, a recent study has shown people tend to reward themselves with some extra junk food.

Uma R. Karmarkar told Harvard Business Review in an interview there's a precedent for this kind of thing:

"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."

Karmarkar and colleague Bryan Bollinger retrieved mounds of data from customer loyalty cards from one location of a California grocery-store chain. After analyzing close to 1 million transactions, the researchers zeroed in on shoppers who brought their own bags (signified by a discount subtracted from the bill). The researchers cut out any transactions that weren't, what they considered, part of a weekly shopping trip.

They found that those who used reusable bags tended to continue their do-gooder trends. People who brought reusable bags were 0.25 percent more likely to buy organic. However, for every good deed there should be a reward, right? So, those who brought reusable bags were also 1.25 percent more likely to add chips and candy to their shopping cart.

The researchers admit that these numbers aren't terribly high, but it's a good example of what consumer psychologists like to call “licensing.” You do something good, so now you can do something bad. It's interesting, however, as “the licensing elements of these results are highly dependent on that motivation arising from the shoppers making a choice for themselves, rather than being directed into it by others,” which grocery stores have been known to do. Food manufacturers often manipulate the shape of containers to make it more appealing to pick up and buy. But this choice is without coercion on any company's part.

Read more about the study at the Harvard Business Review and read the full study here.

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