Bringing a reusable bag to the grocery store is good for the planet. But it's bad for you.

Bringing a reusable shopping bag to the grocery store is a great way to be environmentally friendly. But there may be one unexpected downside.

A new study found that shoppers who bring reusable bags to the grocery store may feel so good about their duty to the environment that they reward themselves — often with the purchase of junk food. Shoppers who bring their own bags are 7 percent more likely to buy junk food than those who don't.

Researchers think the uptick in junk food purchases can be explained by the "licensing effect," which The Atlantic says "describes the phenomenon that after people make a decision they consider responsible and noble, they feel justified in giving themselves a reward down the line."

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With plastic bag bans on the rise across the U.S., cookies might really start flying off the shelves.