By Robert J. Shiller

New York Times News Service

Amid the constant turmoil in domestic and global politics these days, the economy’s steady expansion has been a source of comfort. But look more closely and you will find that economic growth rests on a surprisingly amorphous base: consumer confidence.

In the United States, consumer confidence has been ascending since 2009. What’s more, the domestic data have been synchronized to a remarkable degree with similar metrics from around the world.

We know that consumer confidence is a critically important advance indicator of economic booms and busts. At the moment, it is forecasting a continuing expansion. Yet the troubling fact is that we don’t fully understand how and why consumer confidence acts as it does.

Since 2009, there have been ups and downs in consumer confidence lasting months, but those downward swings have not interrupted the long uptrend in the United States. This pattern has held for all four major U.S. indexes: the University of Michigan’s consumer sentiment survey, The Conference Board’s consumer confidence index, the Bloomberg Consumer Comfort Index, and the Organization for Economic Cooperation and Development’s consumer confidence index.

Some recent readings have been a bit weak, but we have seen such minor shifts many times since 2009 and they haven’t meant much. Even the recent shutdown of the U.S. government — and the prospect of further political conflict — may not matter for this metric. When the government shut down briefly in October 2013, there was a temporary drop in confidence, but that didn’t stop the longer-run rise.

The simplest explanation is the necessarily slow but consistent recovery from the crushing financial crisis of 2008 and 2009. After all, a financial crisis of that magnitude leads to lawsuits, bankruptcies, career disruptions and other economic events that muddy a recovery, and they take a long while to clear up.

There is some truth to that explanation, but it omits a critical yet elusive factor: animal spirits. John Maynard Keynes popularized that term in 1936, referring to a psychological state in which people get a consumerist and entrepreneurial bug that allows them to forget their worries and let their optimism guide their economic decision-making. In homage to Keynes, the economist George Akerlof and I used “Animal Spirits” as the title of a book in 2009. Flourishing animal spirits involve complacency, a playful mood, a “damn the torpedoes, full speed ahead” feeling of confidence.

That kind of exuberance now seems to be fueling the stock market, where prices have outstripped fundamental valuations. Real (inflation-corrected) corporate earnings per share for the Standard & Poor’s 500-stock index were, for the third quarter of 2017, only 6 percent higher than they were in the second quarter of 2007, just before the financial crisis. In contrast, real stock market prices were 39 percent higher. That disproportionate increase is based much more on how earnings are being valued than on how the level of earnings has increased.

While Donald Trump’s presidency may have exerted some impact on animal spirits in the past year, it doesn’t explain the preceding eight years of slowly improving confidence. And I am skeptical that the upward swing can be entirely explained by factors such as government and central bank stabilization policy or technological innovation.

Scholars are working on this issue. At the American Economic Association meeting in Philadelphia this month, I led a session on “Confidence, Animal Spirits and Business Cycles.” All three researchers presented papers describing how animal spirits are helping to drive the economy in the United States and across the world.

Ayhan Kose, director of the World Bank Group’s work on global macroeconomic outlook and forecasts, summarized work by the bank’s researchers on confidence indexes and the business cycle. They compiled a new database of such survey-based indexes and measures of economic activity in 95 countries. Much of their data goes back before 2000, and in some cases to the early 1960s. The researchers found that consumer and business confidence tended to start declining before the peak of the business cycle, and to start recovering before recessions ended.

In short, confidence indexes are indeed leading indicators in a vast array of countries, and these individual country confidence indexes are driven by a global cycle, though how this cycle is synchronized still isn’t clear. Fluctuations in animal spirits remain an essential mystery for economists, though it is encouraging to see that quantitative studies of the issue are underway.