The U.S. Social Security retired worker benefit calculation is based on the average of the highest 35 years of each individual's earnings; thus, payroll taxes for people with flat or declining earnings can effectively become a pure tax near the end of their working careers. Individuals who still have zero or low–earning years being factored into their high–35 calculation face much lower (even negative) effective tax rates if they work additional years.
What is the Effective Social Security Tax on Additional Years of Work?John Sabelhaus ,
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National Tax Journal