So someone has asked you to co-sign for a student loan.

Chances are, it’s your child or grandchild, or perhaps a niece or nephew. You have unrelenting faith in this teenage freshman, or near certainty that graduate school will lead to a lifetime of gainful employment. And maybe you feel badly that the family has not been able to save enough to pay the bills outright.

Fine. But be very, very careful.

When you co-sign for a loan, you, too, are responsible for it. If the primary borrower can’t pay, you have to. If that borrower pays late, your credit could get nicked as well. And the mere existence of the loan on your credit report may keep you from being able to get other kinds of loans, since lenders don’t always want to do business with people who already have a lot of debt.

In some cases, the lender will try to collect from a co-signer even if the primary borrower is dead, as a recent collaboration between ProPublica and The New York Times revealed. Legislators in New Jersey held hearings on the matter this week.

After a postrecession lull, the so-called private loans — which generally have less favorable rates and terms than federal loans, and tend to require co-signers — are making a comeback of sorts. About one in 10 undergraduates takes one out, according to Sallie Mae, the biggest lender. Undergraduate and graduate students together borrow $10 billion to $12 billion in new private loans each year, according to MeasureOne, a market research and consulting firm, and the trajectory has been upward since the 2010-11 school year.