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It's harder to overspend when you carry cash.
"You will spend less money when you know that, when the money in your pocket's gone, you're done," says Larry Winget, author of "The Idiot Factor: The 10 Ways We Sabotage Our Life, Money, and Business."
Unlike credit or debit cards, which can go over the limit or into a negative balance, "you can't slide past zero when you carry cash," he says.
"And that's how you get in trouble," Winget adds. "Plastic, when you use it, doesn't show you're out of money."
There's an upside to a full wallet. "I believe that it makes you feel more prosperous when cash is in your pocket," Winget says. "You just feel better when you have money. You never feel broke -- which is a sense of independence and power."