Stocks around the globe tumbled on Monday, extending a sell-off that began last week. Investors, who had been riding a wave of optimism last year, have recently shown nervousness as global economic conditions have been shifting.

The S.&P. 500 had its biggest one-day drop in more than six years

■ The Standard & Poor’s 500-stock index fell more than 4 percent, erasing its gains for the year. The 4.1 percent drop was the worst for the S.&P. since August 2011.

■ The Dow Jones industrial average fell more than 1,000 points, its biggest point drop ever. But that record is a function of the Dow’s size, which is the largest it has ever been. In percentage terms, it fell 4.6 percent, something it also did in 2011.

■ Bond yields have risen fast in recent weeks as people have moved money out of stocks into these traditional safe havens. Yields were down Monday from a 4-year high on Friday.