On Sept. 20, stock investors in the United States were sitting on a nearly 10 percent gain for the year. Benchmark American crude oil prices were up more than 20 percent. The tech-heavy Nasdaq composite index was up more than 15 percent.

But those profits are gone.

Since October, an index of commodities that includes oil and copper has swung from gains of 20 percent to losses for the year. A similar move took place in risky corporate bonds. So far this year, after another steep drop on Friday, the S&P 500-stock index is down about 2.8 percent.

[Read more: The stock market has wiped out its 2018 gains. There have been several moments like this in the last decade.]

“In a typical year there’s going to be some winners and losers,” said Ed Clissold, chief United States strategist at the equity market research firm Ned Davis Research. “It’s very rare that you get nothing working.”

His firm recently looked at eight types of investments going back to 1972. In every year, at least one of these categories generated a return of 5 percent or more. A separate study by JPMorgan Chase analysts found that “2018 has delivered losses on almost every asset class and investing style.”

The widespread market jitters have accompanied a tectonic shift in the investing world, as central banks have begun to withdraw the extraordinary support provided to the global economy in response to the financial crisis a decade ago.

For nearly seven years, the Federal Reserve kept interest rates near zero and bought trillions of dollars’ worth of government bonds, which pushed interest rates — that move in the opposite direction of prices — sharply lower. Money in the bank earned next to nothing, so investors eagerly bought anything they expected to generate some kind of return: risky debt, real estate, stocks, technology start-ups.