It depends, but in most cases, yes. More than 9 of 10 private student loans taken out by undergraduate students are co-signed. Graduate students are more likely to take out loans without one. You do not have to add a co-signer unless you are under the age of majority in your state (usually between 18 and 21). However, if you have a limited or poor credit history or a poor credit score, a lender may require you to add a co-signer to balance out their risk on the loan. Adding a co-signer with good credit can improve your chances of qualifying for a private student loan. In general, the better your co-signer’s credit, the better the rates you can qualify for. Credible makes it easy to compare co-signers to see which co-signer can get you the best rate.