The Social Security is one of the main public pension system for retired Americans. Despite the presence of contribution-based retirement plans such as 401Ks, individual retirements plans, widespread popularity of equity investments including mutual funds, the good old social security payment from Uncle Sam supports a big portion of the living expenses of retired people. In fact, about half of seniors depend on social security for about 50 percent of their income. Social security benefits in the US are lower than many other developed countries according to a study by the Center on Budget and Policy Priorities.

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Source: Social Security Benefits Are Modest, CBPP

The social security system in the US replaces about 40% of the earnings of a worker. The average OECD country has a public pension program similar to that of the US but covers half of the earnings.