When Social Security was born in 1935, the SSN was created to keep a record of a worker's earnings and later to monitor the benefits paid to that worker. But over the years, the SSN has become the most widely used way for government and businesses to identify the individuals with whom they have contact. Today the SSN is at the heart of many record-keeping systems in the United States.

A long list of federal, state and local agencies, starting with the Internal Revenue Service, is authorized by the Privacy Act of 1974 to request your SSN. You can find the names of some of these agencies at this Social Security website.

In those cases, the law requires the agency to tell you its authority for requesting your SSN, whether disclosure is mandatory, what use will be made of your number and the consequences, if any, of failure to provide the information.

When a business requests your SSN, you can refuse to give it — although you might then not be able to make the purchase or get the service you want.

If you're uncertain what to do, find out why the number is needed and how it will be used — then decide whether you want to disclose it. You might be able to persuade the business to confirm your identity through some other means, such as a driver's license.

Also of interest: A fraud alert or a credit freeze. >>

Stan Hinden, a former columnist for the Washington Post, wrote How to Retire Happy: The 12 Most Important Decisions You Must Make Before You Retire. Have a question for the Social Security Mailbox? Check out the archive. If you don't find your answer there, send a query.