Apend<$>change to a popular credit scoring system is about to it make it much harder for people to polish their credit by riding the coattails of someone else's good payment record.Fair Isaac Corp., the creator of the widely used FICO credit scoring formula, is adjusting its scoring method to fight a growing trend called piggybacking.

Piggybacking involves letting another person become an authorised user on your credit card account. As an authorised user, that person immediately inherits the payment history of that account. People seeking to boost their credit ratings typically hitch themselves to folks with histories of paying their cards on time and keeping their balances low, all of which maximises their credit scores.

In the past, this arrangement has been between family members or friends. Parents are often advised to do it for their college-age or young adult children (a move I highly discourage).

In recent years, a growing number of companies have begun selling authorised user services. These so called credit-repair companies, which advertise heavily on the internet, promise that people can push up their scores by 200 points or more. FICO scoring ranges from a low of 300 to a high of 850. The higher your score, the better the loan terms and rates you can get.

Brokers selling this service also promise that authorised users won't get the credit card number or information that could allow them to use the credit card. However, if the information is included on a person's credit report, which is the whole point of this transaction, it's possible to find a way to use the card.

Users of a commercial piggyback service might pay $1,000 or more depending on quality of the accounts. Account holders who let others be added to their accounts may earn a few hundred dollars per piggyback client. The middleman collects the difference.

There's a problem with this service. It allows people, who have no relationship with the primary cardholder, to embellish their credit histories, making it possible for them to get loans for which they would otherwise not qualify.

"This is clearly loan fraud," said Craig Watts, the public affairs manager for Fair Isaac.

Piggybacking has become a major concern to lenders and banking regulators, who are already dealing with historically high mortgage loan delinquencies and foreclosures. The damage could get worse if they approve borrowers who only have an illusion of creditworthiness.

By September, Fair Isaac will begin rolling out its new scoring model, called FICO 08. Watts said consumers relying mostly on the credit history of another cardholder could see their scores drop if the new scoring model is used. Fair Isaac estimates about 25 to 30 percent of credit reports have at least one authorised user.

"While FICO's move has largely remained under consumers' radar screen, its impact will be clearly felt when the change starts taking place in September, particularly among newly divorced women and a fresh crop of college students who will face a new hurdle in establishing credit for the first time," says John Ulzheimer, a former FICO manager and current president of Credit.com educational services.

No question, piggybacking has helped some consumers. But borrowing someone else's good credit history doesn't always have a happy ending.

I've heard my share of horror stories from people who allowed a relative or friend to become an authorised user only to be left with major debt. Or the authorised user, who was responsible for paying the bill, didn't make payments on time, thus ruining the primary cardholder's good credit name.

Being an authorised user doesn't give someone a chance to demonstrate that they can actually use credit wisely. In fact, that's why in creating its own credit scoring system, VantageScore, the three major credit bureaus never included authorised users in calculating a person's creditworthiness, according to Lisa Zarikian, who leads Equifax Predictive Sciences and was a member of the team that created the scoring system.

Although Fair Isaac is updating its scoring model, piggybacking will continue for now because not all lenders will immediately change over to the updated FICO version.

I understand the desire to want to help someone qualify for credit or get the best loan deal. But that person's inability to qualify for credit could be a good thing. Just because someone wants credit doesn't mean they can afford it. That's why they didn't qualify on their own in the first place. Maybe they aren't ready for the responsibility.

And consider this. If you never meant to allow an authorised user to use your card, you're complicit in helping someone misrepresent themselves as creditworthy. Bottom line: this is unethical whether it's being done by somebody's mama, daddy, friend or stranger.

Listen to Michelle Singletary discuss personal finance every Tuesday on NPR's "Day to Day." To hear her reports online go to www.npr.org. Readers can write to her c/o The Washington Post, 1150 15th St., N.W., Washington, D.C. 20071. Her e-mail address is singletarym(at)washpost.com. Comments and questions are welcome, but due to the volume of mail, personal responses may not be possible. Please also note comments or questions may be used in a future column, with the writer's name, unless a specific request to do otherwise is indicated.