At a time when central banks are holding interest rates at record lows, the return from holding plain vanilla corporate bonds is negligible, so investors are more willing to buy junk, for the higher yields. And this hunt for higher returns has been playing out worldwide as asset managers chase higher returns anywhere they can be found, whether in United States private equity or in the bond markets of Europe and emerging economies like Argentina and Turkey.

The biggest risks outside the United States are in China, which has printed by far the most money and issued by far the most debt of any country since 2008, and where regulators have had less success reining in borrowers and lenders. Easy money has fueled bubbles in everything from stocks and bonds to property in China, and it’s hard to see how or when these bubbles might set off a major crisis in an opaque market where most of the borrowers and lenders are backed by the state. But if and when Beijing reaches the point where it can’t print any more money, the bottom could fall out of the economy.

More broadly, the trigger to watch is the United States Federal Reserve, since many other central banks in the world tend to follow the Fed’s lead in setting interest rates. Over the last 50 years, every time the Fed has reined in easy money by raising interest rates, a downturn in the markets or the economy has followed eventually. It may take a while, but trouble almost inevitably does come.

Many doomsayers worried that the Fed tightening that began in 2004 would help prompt a recession — and it eventually did, in 2008. Though rates are still historically low in the United States, the Federal Reserve began to raise them more than two years ago and is expected to continue tightening them into next year.

The Fed’s tightening is already rattling emerging markets. When the American markets start feeling it, the results are likely be very different from 2008 — corporate meltdowns rather than mortgage defaults, and bond and pension funds affected before big investment banks.

If a downturn follows, it is more likely to be a normal recession than another 100-year storm, like 2008. Most economists put the probability of such a recession hitting before the end of 2020 at less than 20 percent.