Your credit score, like your fingerprint, is your own and no one else's. But if you hold a joint credit account, have cosigned a loan or have been authorized to use another person's credit, those things can appear on your own credit report and impact your credit score.
It's important that joint account holders understand that their credit behavior will affect the other joint account holder. The same thing is true if you are an authorized user on someone else's account. What you do, and how you pay (or don't pay) your bills will affect your joint account holder's credit.
Let's say your cousin wants to buy a car and you agree to cosign the auto loan. The auto loan will get listed on your credit report and on your cousin's credit report. If for some reason your cousin stops making on-time payments each month, your credit report will take a hit, even if your name isn't on the car's title. Not only that, but the lender will come looking for you to make the payments, even if the car is repossessed.
Your credit score cannot be affected by activity on an account held solely in the name of your spouse, partner or child. However, in some states (Arizona, California, Idaho, Louisiana, Nevada, New Mexico, Texas, Washington and Wisconsin), all debt acquired during a marriage is considered a joint debt—regardless of whether the account is a joint account, in your name alone or in the sole name of your spouse.