But as the market fell from its 2000 peak, the confidence index rose to high levels again, where it remained until recently. This year, the index took a dive for individual investors, and now is at the lowest level since 2000. The confidence of professional investors has fallen, too, but not as sharply. In short, people are beginning to worry.

Yet valuations remain high, and it would be comforting if they made sense. So I’ve been trying to come up with a theory to explain today’s elevated stock prices — and maybe convince myself that they could remain lofty for some time. One factor to consider is that bond prices are high, too.

Inflation is running at only around 2 percent, and 10-year Treasury notes yield less than 2.5 percent, a very low level. Bond prices move in the opposite direction as interest rates, and high bond prices may account for the high valuations in stocks.

The inverse was certainly true in the late 1970s and early ’80s, when inflation and Treasury yields were in the double digits. Back in 1979, Franco Modigliani of M.I.T. and Richard A. Cohn of the University of Hartford argued that investors were then undervaluing the stock market, because they failed to perceive that despite high inflation, real interest rates were relatively modest. The two men rightly predicted that there would be a stock market boom if inflation fell significantly — as we now know it began to do within a year after they made their case.

It’s possible that bond prices account for today’s stock market valuations. But that raises another question: Why are bond prices so high? There are short-term explanations: the role of central banks, for example. But is there a compelling reason for prices of stocks and bonds (and maybe houses, too) to remain high indefinitely?

I’ve looked for untraditional answers. Perhaps today’s prices have something to do with anxiety about the future. I suspect that after the financial crisis, working people are much more worried about their future pay. Many are concerned that they might lose their jobs to cost-cutting, or that they might eventually be replaced by a computer or robot or website. Such anxiety might push them to try to make up for these potential shortfalls by investing in stocks and bonds — even if they worry that these assets are overvalued.

Extrapolating from a theory of Robert E. Lucas Jr. of the University of Chicago, one might well expect lofty stock prices amid such worries: When there aren’t enough good investing opportunities, people wishing to save more for the future may succeed only in bidding up existing assets even if they think they’re overpriced. Call it the “life preserver on the Titanic” theory.