We're a nation of people who like to plunk our credit cards on the counter in front of the cashier. How and why we use those number-stamped pieces of plastic can vary pretty dramatically.

Consider this: Almost six in 10 adults count a credit card bill as a regular monthly expense, along with the mortgage, utilities, and other bills. That's what The Pew Research Center found in a recent survey, which also discovered we're almost evenly divided in our credit card habits.

About a quarter of us can pat ourselves on the back for paying the bill in full every month. However, another three in 10 people don't -- they make a payment toward the balance on their cards. (About one in 10 of us, by the way, are also paying off store financing for home furnishings or electronics.)

You know what's coming next -- the Credit Card Lecture. Actually, you can probably deliver this lecture to yourself; you don't need me. You know it's not a good idea to carry a balance on your credit cards, yet you do it anyway. One reason might be that once the balance accumulates, you think of it as a regular monthly expense -- just like the mortgage, gas, and electricity.

Let's go back to the beginning for a moment. You have a credit card balance because you purchased something using that card. You stood in the store, looked the merchandise over, and probably decided you could have it for a fair price (or, at the very least, a price you were willing to pay).

When you pay for items with a credit card, what happens to your fair price? Interest drives the cost of all those goods up. Put $5,000 of seemingly reasonable purchases on a credit card charging 18% interest, and you'll pay almost $300 extra if you pay everything else in a year.

The math really turns against you if you only pay the minimum on your card each month. You'll almost double your costs by paying $4,800 in interest over virtually 19 years that it takes to kill that balance. Your great buys aren't such a bargain anymore.

You probably already know that using a credit card when you can't pay the balance isn't a great idea. So, why do we persist in this very foolish and very un-Foolish behavior? The Pew survey asked all its respondents why they used a credit card. Here's what they said and why it might lead you astray:

  • Two-thirds of us say paying by credit is more convenient. In fact, paying by credit doesn't just speed you through the checkout line, it also prevents you from thinking too much about your purchase. Pay with a debit card, and you have to remember whether there's enough money in the bank. The same goes for writing a check. Pay with old-fashioned dollars and you might even think twice before handing over your cash.
  • About six in 10 people say they use credit cards to finance unexpected expenses. This can be another major convenience of using a credit card, especially if you're stuck on a highway with a broken-down car and no other way to pay for a tow truck. Here's where a healthy emergency fund can bail you out. Even if you pay for your unexpected expenses by credit, you'll want to pay for them as soon as the bill arrives. A well-stocked emergency fund can prevent you from accumulating a big balance on your card.
  • Looking to buy a new refrigerator? About 37% of us say we like to use our credit cards to finance a major purchase. That might not be the worst decision if your card has a rock-bottom interest rate. Chances are yours doesn't. In either case, you save money by saving up for your major purchases before buying. Interest payments just make your major purchases more expensive.
  • About one-third of us say we use credit cards for the points, miles, or the other perks of a reward program. If you're paying off your balance, and your card doesn't charge an annual fee, you're probably getting the best of your card. If you're rationalizing your excessive purchases and large balance by telling yourself you'll be able to fly to the Bahamas next winter, then you have a problem. Perks virtually never pay off when you're carrying a balance. You could just put all that extra money toward your goal.
  • Approximately 30% of us use credit cards for what might the worst reason around -- we buy stuff when we run out of money at the end of a pay period. Charging after you've run out of money can become an increasingly deep pit that's hard to get out of. It's time to sit down and rework your spending to make sure you don't run out of money before you run out of necessary expenses.

If you're running on the credit card treadmill and just can't seem to escape, you can find help at the Credit Center. While there, you'll find a free guide to help you get out of debt and join those people who pay their bills in full each month.

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Fool contributor Mary Dalrymple welcomes your feedback. The Motley Fool has a disclosure policy.