BOSTON — HOUSEHOLDS can’t spend, on a continuing basis, more than they earn. Countries can’t either, at least not over the long run. But countries can certainly leave the bill for their current spending to the young and to future generations. Official borrowing is the old-fashioned way to do this: Sell Treasury bonds, and other securities, and spend the proceeds. But borrowing in broad daylight has a drawback: The more you do it, the more lenders worry about repayment, and the more interest they charge for their loans.

So there’s a different way to borrow — one that’s more subtle, harder to see and, therefore, cheaper to do. You still take in money, pledge to return it, and leave future generations on the hook. But you call the money you take in “taxes,” not “borrowing.” And you promise repayment in the form of pension, health care and other benefits, but you don’t record the present value of those promises as official debt.

Social Security is a prime example. It takes in money, via payroll taxes, while promising hefty retirement benefits in return. Dig deep into the appendix of the most recent Social Security Trustees Report, released on Monday, and you’ll find that the program’s unfunded obligation is $24.9 trillion “through the infinite horizon” (or a mere $10.6 trillion, as calculated through 2088). That’s nearly twice the $12.6 trillion in public debt held by the United States government.

Social Security is backed by something perhaps even more powerful than the full faith and credit of the government: the political power of some 100 million Americans 50 and older.