For much of the 20th century, Americans sought security in the form of guarantees from large organizations. The federal government would provide Social Security benefits for workers and their dependents. Large corporations would provide defined-benefit pension plans promising specific payments to retired employees. The assumption was that experts at the top of these big organizations could use scientifically obtained knowledge to take better care of us than we could take care of ourselves.

Now there’s reason to challenge that assumption. The experts, it turns out, are fallible. The Social Security system faces rising gaps between revenues and promised benefits starting in 2017 and an exhaustion of trust fund assets in 2041. Meanwhile, large corporations – the latest is United Airlines – have defaulted on their defined-benefit pension plans and passed them off to the government sponsored Pension Benefit Guaranty Corp., which does not pay all of the promised amounts.

The Social Security system’s problems are, in large part, the product of increases pushed through Congress by House Ways and Means Chairman Wilbur Mills way back in 1972 and signed by President Richard Nixon, who was running for re-election. The increases had the good effect of wiping out poverty among the elderly. The experts of the day argued that they would be paid for by the offspring of the baby boomers then reaching adulthood.

But the baby boomers didn’t produce a second baby boom. In 1983, legislation cutting benefits and raising taxes solved the problem for a time. But now it looms squarely ahead.

Not to worry, say opponents of George W. Bush’s proposal for personal retirement accounts. The federal government guarantees the benefits. But it doesn’t. In 1960, the Supreme Court, in Flemming v. Nestor, ruled that there was no right to Social Security benefits. Social Security, wrote Justice John Harlan, “was designed to function into the indefinite future, and its specific provisions rest on predictions as to expected economic conditions, which must inevitably prove less than wholly accurate, and on judgments and preferences as to the proper allocation of the nation’s resources, which evolving economic and social conditions will of necessity in some cases modify.”

The Pension Benefit Guaranty Corp. was set up by the ERISA pension law in 1974 and is financed by fees from companies with defined benefit pension plans. Unfortunately, as of September of last year, the PBGC’s program funds had a net deficit of $23.5 billion. The PBGC’s biggest obligations are for the pension funds of bankrupt steel companies (Bethlehem, ltv, National) and airlines (TWA, US Airways, and now United), all of which had strong unions that negotiated big benefits that the weakened companies proved unable to fund.

Now there is talk that General Motors and Ford may have trouble financing their huge defined-benefit plans. The practical result is that companies with fully funded pension plans will have to pay higher PBGC fees to bail out those without. The lesson is that experts can err and big organizations can’t always be relied on. When they fail, they fail big.

Almost all investment advisers routinely tell their clients to diversify their portfolios to reduce risk. Reliance on large organizations may spread benefits, but it also concentrates risk.

As the 20th century progressed, the private sector gravitated toward such diversification. Since 1980, the number of workers with defined-benefit pensions has decreased, while the number with defined-contribution pensions – which the employer partially funds and the employee owns and invests – has increased. Personal retirement accounts as part of Social Security would allow lower-income workers, who typically have little in the way of investments, to accumulate wealth over the course of a working lifetime, as most Americans do.

Some Democrats attack this plan as hugely risky, because the stock market goes up and down. But the stock market has always gone up over the course of a typical lifetime. The Democrats’ more serious argument is that Social Security should be a bedrock guarantee not subject to market risk. But this is antiprogressive: It leaves lower-income workers with less ability to accumulate wealth.

More important, what if the bedrock turns out to be quicksand? The experts, as Justice Harlan noted nearly a half century ago, don’t always get it right.