Social Security Social Security is a defined-benefit pension plan managed by the government. The government collects "contributions" in the form of social security taxes. It pays benefits according to formulas set by Congress. As with any defined-benefit plan, there is no assurance that the value of what you pay in will equal, exceed, or fall short of what you could have obtained had you taken your contributions and saved them. On average over the history of Social Security, the benefits have been greater than they would have been under a defined-contribution allocation. That is a political decision made by Congress, not an economic result.
At any one point in time, Social Security transfers money between two groups of individuals. Taxpayers pay into the system, and beneficiaries take money out of the system. Social security's financial condition depends on the balance between these two groups. In 1950, when the system was relatively new, there were 16 workers per retiree. Even now, the ratio of taxpayers to beneficiaries is 4.0, which is relatively high. This gives Congress the option of collecting more in taxes than it pays in benefits, with the surplus going to fund other government spending or to pay down government debt.
As the Baby Boomers age, however, the ratio of taxpayers to beneficiaries will fall. According to a report on global aging by Maureen Culhane of Goldman Sachs (my link to it is broken, but you may be able to Google it), this ratio will fall to 3.4 by 2015 and to 2.3 by the year 2050. (In fact, it is likely that the low point for the ratio will come somewhere in between 2030 and 2040, when it may fall below 2.0) Other things equal, if the ratio is 3.0 instead of 4.0, in order to maintain benefits, each taxpayer must pay 4/3 of what he or she would have paid had the ratio remained at 4.0. As longevity increases relative to the statutory retirement age, people remain Social Security beneficiaries for more and more of their lives. Thus, the outlays for Social Security tend to grow faster than the economy. In 1960, social security outlays were 2.2 percent of GDP. In 2000, they were 4.1 percent of GDP.