The grandmother of all middle-class entitlements, Social Security is undoubtedly the most popular government program in America, credited with dramatically reducing old-age poverty. Yet it has a major structural flaw that you don't need to be a whiz to grasp. The people who work and pay into the system are financing the benefits of today's retirees while relying on the next generation of workers to fund their future benefits. However, there are fewer and fewer workers supporting more and more beneficiaries -- both because people are living longer and staying around to collect the checks, and because birth rates fell sharply after 1960. In 1950, the ratio of workers to pensioners was 16 to 1; today, it's 3.3 to 1, and in 25 years it's projected to drop to 2 to 1.

The postwar baby boom followed by the baby bust has obviously worsened the problem. But any pay-as-you-go retirement program will always be at the mercy of such demographic vagaries. Surveys in recent years have found widespread popular support for reforms that would let people invest a portion of their Social Security contributions in the stock market. In a Washington Post/ABC News poll in September, 75 percent of registered voters 18 to 30 years old, 66 percent of those 31 to 44 and 57 percent of those 45 to 60 endorsed such proposals.

Naysayers -- from leftist economist Robert Kuttner to columnist Ellen Goodman -- are wont to dismiss the new enthusiasm for privatization as a myopic, irrational response to the booming economy and the soaring Dow Jones. (Some anti-privatizers greet every market downturn with barely disguised glee.) But when you know how well your money could do in the private market, it's pretty irksome to hand over 6.2 percent of your salary -- 12.4 percent if you count the employer's share -- to Uncle Sam for a promised annual return rate of 2 percent. It's especially galling for self-employed people like me, who have to shell out the entire 12.4 percent out of their own wallets.

In fact, estimates of how retirement savings would have fared in a private system are based on long-term trends, including the downturns and the crashes. Even before the current boom, from 1929 to 1996, the average annual return rate on market investments was about 7 percent.

Besides, the popularity of privatization (which began to show up in the polls in 1994) also has to do with a sense of a looming crisis. That bite taken out of your paycheck really hurts if you doubt you'll ever collect the reward, even at 2 percent interest. According to the latest report from the Social Security Board of Trustees, issued in March, by 2015 revenues from payroll taxes will not keep up with the benefits. To keep the checks coming, the government will have to use the "trust fund," the Social Security surplus accumulated since 1983 -- which, by current estimates, will be empty by 2037. Then, the only way to keep the system alive will be to hike the payroll tax, slash the benefits or both.

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