When you share a credit card with someone, you are effectively giving them access to your wallet and your reputation. You need to be careful about who you grant that access to, and what they are doing with it.

On behalf of CardRatings.com, survey company Op4G asked 2,000 adults about their credit cards. The answers give some insight as to who is signing up for joint accounts and why they do it. For those who have joint accounts, there are some important guidelines for making the arrangement work.

Who's sharing their credit cards?

Joint credit cards are not the norm, but they are fairly commonplace, as 27 percent of those surveyed shared a joint credit card account. Of those with joint credit cards, 81 percent identified their spouse or domestic partner as the joint account holder.

Who else do people share credit cards with? Nearly 12 percent said they shared a credit card with a parent, which is interesting since this survey was conducted entirely among adults. In fact, nearly half of those sharing a credit card with a parent are themselves over the age of 29, suggesting that either those parents or their offspring need some degree of financial assistance well into adulthood. Adults sharing credit cards with their parents was actually more commonplace than adults sharing with their children, which only 4 percent of survey respondents did.

Why are they doing it?

Why are all these people sharing credit cards? The most common reason is to make shared expenses and budgeting easier, which was cited by slightly more than half the respondents as why they had a joint account. Another 31 percent said they had a joint credit card to help themselves or their joint cardholder build or improve credit. Nearly 11 percent said they had a joint credit card to maximize rewards benefits, while the remaining 7 percent did not cite a reason for the arrangement.

As you might expect, people who shared a credit card with a parent or child typically had different reasons for doing so than those who shared a card with a spouse. Making shared expenses and budgeting easier was the reason for having a joint credit card among nearly 58 percent of those who had a joint account with a spouse, while this reason was cited by just 12.5 percent of those who shared a card with a parent or a child. Instead, most people sharing a card with a parent or a child were doing it to help build or improve the credit of one or both of the parties.

Among spouses, men and women have different recollections of how they came to the joint account arrangement. 70 percent of women think it was a mutual decision, with just 19 percent believing it was their idea alone. Among men though, just 48 percent believe it was a mutual decision, while 40 percent believe they initiated it. This may be an example of how men like to believe they are in control of financial decisions -- or of how women are skilled at making men believe they made those decisions.

Making it work: Tips for effectively sharing

Whoever you share a credit card with, and for whatever reason, here are some guidelines for making the arrangement work.

  1. Set parameters. Go into the arrangement with a clear understanding of the spending limit, who is going to pay for what and how the balance is going to be kept under control.
  2. Monitor statements. Even if you aren't the person paying the bill, look at every statement. Regardless of whose fault, you both are financially responsible if the card runs up debt, and the results will go on both your credit histories.
  3. Measure the cost/benefit of rewards. Rewards credit cards typically come with higher interest rates. This is a good deal if you pay off your balance every month so you don't pay interest, but if you carry a balance that incurs interest charges, you have to figure out whether the rewards you use are worth the extra interest you pay.
  4. Don't be afraid to pull out. If your joint account holder isn't acting responsibly, pull the plug before there is serious damage to your finances and credit history. The safest thing is to cancel the card altogether, because trying to pull out of a joint credit arrangement individually is difficult.

Joint credit cards introduce some complications, but many people are able to make them work. How you handle the arrangement will make all the difference in determining whether a joint credit card is a shared resource or a shared problem.