Who doesn’t know that junk food is bad for you? It’s empty calories, puts on the pounds and is awfully, terribly, almost impossibly hard to resist.

It turns out, though, that you’ll buy less junk food if you put away your credit (or debit) card and always pay cash.

That’s the finding of a Cornell University study in the Journal of Consumer Research.

Unhealthy foods — cookies, potato chips, etc. — are impulse buys, says study author Manoj Thomas, an assistant marketing professor at Cornell. "The pain of paying in cash can curb impulsive urges to purchase such unhealthy food products," he writes.

We buy fewer of these "vice" foods if we’re paying with real money instead of plastic.

"When you pay in cash, there is something that makes you feel bad — to part with money," says Thomas. And, his study shows that "when you feel bad about paying with cash, you start paying more attention to the healthfulness of the food. You start asking yourself, ‘Is this healthy food? Should I be buying it?’"

Participants in one shopping test bought 42 percent more junk food ($14.07) when they paid with a credit card than those paying cash ($9.89). The method of payment didn’t make a difference on good-for-you groceries. Both the credit and cash groups spent about $17.50 on "virtue" products like oatmeal and fat-free yogurt.

Thomas isn’t saying everyone should stop using credit cards to buy food. But for people who have trouble resisting junk food, "going to the bank, getting cash might actually be worth it because it will help them be more healthy."