Social Security provides retirement benefits to more than 47 million retired workers at the end of their careers, as well as provides support to family members of those workers. The vast majority of workers earn benefits by paying into the Social Security system through payroll taxes.

Yet what many people don't realize is that there are some workers who aren't part of Social Security. They don't have to pay Social Security payroll taxes on their earnings during their careers, but they also can't count on the program to provide retirement benefits. In particular, teachers and other public sector workers in 15 states across the country get left out of Social Security in many cases.

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