“If you’re invested only in bonds and you’re withdrawing 4 percent, plus inflation, your portfolio will decline,” said Maria A. Bruno, senior investment analyst at Vanguard. “That’s why we recommend that most people hold some equities. And why it’s important to be flexible.” In some years, investors may need to withdraw less than 4 percent, she said, and in some years they can take more.

Clearly, such flexibility depends on individual circumstances. Billionaires can afford to be very flexible: just 2 percent of a $1 billion portfolio is still $20 million. With economizing, even a big spender should be able to scrape by on that. But $20,000 — the cash flow from a $1 million portfolio at 2 percent — won’t take you very far in the United States today.

And if you’re not close to being a millionaire — if you’re starting, say, with $10,000 in financial assets — you’ve got very little flexibility indeed. Yet $10,890 is the median financial net worth of an American household today, according to calculations by Edward N. Wolff, an economics professor at New York University. (He bases this estimate on 2010 Federal Reserve data, which he has updated for Sunday Business according to changes in relevant market indexes.)

A millionaire household lives in elite territory, even if it no longer seems truly rich. Including a home in the calculations, such a family ranks in the top 10.1 percent of all households in the United States, according to Professor Wolff’s estimates. Excluding the value of a home, a net worth of $1 million puts a household in the top 8.1 percent. Yet even such families may have difficulty maintaining their standard of living in retirement.

“The bottom line is that people at nearly all levels of the income distribution have undersaved,” Professor Wolff said. “Social Security is going to be a major, and maybe primary, source of income for people, even for some of those close to the top.”

Professor Munnell said that in addition to relying on Social Security, which she called “absolutely crucial, even for people with $1 million,” other options include saving more, spending less, working longer and tapping home equity for living expenses. “There aren’t that many levers we can use,” she said. “We have to consider them all.”

THE bond market has always been a forbidding place for outsiders, but making some sense of it is important for people who rely on bond income.