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Social Security is an income transfer program. It takes money from the working generation and uses it to provide for pensions for the retired generation. Special mathematicians called actuaries calculate how much tax is needed to fund these benefits.
For the last 25 years, American social security has run a surplus. That means, the program has collected more than it needed to to pay current beneficiaries. The excess money was put into real, United States government debt called bonds—which, in turn have paid interest. When social security needs the money from those bonds, the social security administration will present those bonds to the US treasury, and the treasury will use the then-current tax monies to satisfy the bonds.
Under one pessimistic scenario, the social security surplus will run dry in 2033, and the amount of contributions (FICA tax) will only be sufficient to provide for 75% of the expected check. This does not mean social security will be “bankrupt”. It will mean that taxes will be raised to support the promised benefit. Or, it will mean that the benefit will be reduced under some formula.
There are some people, groups small and very loud, who do not like social security. Don’t listen to them—they just don’t like the program. Social Security works because it has been designed to last. It’s lasted 77 years this week, and will be around, easily, 77 more.