A decade and a half ago, both of us served on President Clinton's Bipartisan Commission on Entitlement and Tax Reform, the forerunner to President Obama's recent National Commission on Fiscal Responsibility and Reform. In 1994 we predicted that, unless something was done to control runaway entitlement spending, Medicare and Social Security would eventually go bankrupt or confront severe benefit cuts.

Eighteen years later, nothing has been done. Why? The usual reason is that entitlement reform is the third rail of American politics. That explanation presupposes voter demand for entitlements at any cost, even if it means bankrupting the nation.

A better explanation is that the full extent of the problem has remained hidden from policy makers and the public because of less than transparent government financial statements. How else could responsible officials claim that Medicare and Social Security have the resources they need to fulfill their commitments for years to come?

As Washington wrestles with the roughly $600 billion "fiscal cliff" and the 2013 budget, the far greater fiscal challenge of the U.S. government's unfunded pension and health-care liabilities remains offstage. The truly important figures would appear on the federal balance sheet—if the government prepared an accurate one.

But it hasn't. For years, the government has gotten by without having to produce the kind of financial statements that are required of most significant for-profit and nonprofit enterprises. The U.S. Treasury "balance sheet" does list liabilities such as Treasury debt issued to the public, federal employee pensions, and post-retirement health benefits. But it does not include the unfunded liabilities of Medicare, Social Security and other outsized and very real obligations.