Analysis of information sources in references of the Wikipedia article "Federal Insurance Contributions Act" in English language version.
[T]he progressivity of the tax structure cannot be judged by looking at only one component of taxes.... In recent years the fastest-growing component of federal taxes has been the payroll tax, which is regressive (the opposite of progressive) in its impact, because it taxes at a flat rate only on wages below $63,400 (in 1991). The Social Security system, however, is progressive because it pays higher benefits—relative to taxes paid in—to lower-income workers.