Trading volumes have fallen as major indexes climb toward all-time highs, the latest sign that investors remain cautious despite 2019’s powerful stock rebound.

Last week, the S&P 500 rose to a fresh six-month high and pulled closer to a record. Trading was also quiet the week prior even as the index rose.

Analysts say that trend reflects investor caution as uncertainty about trade talks and mixed economic insights affect the outlook for stocks. According to EPFR Global, billions of dollars continue to flow out of stock mutual and exchange-traded funds.

Low trading volumes worry market watchers because analysts say they can create market swings in both directions if momentum changes.

“This has always been that critical quarter to see, ‘Did it really make a difference or not?’” said Chief Executive Officer of ZEGA Financial Jay Pestrichelli. “There’s a lot of folks waiting to see what the real deal is.”

S&P 500 companies are expected to post their first drop in quarterly earnings in nearly three years, and some investors are waiting to see what happens to corporate profits as the effect of tax cuts passed in 2017 fades.

After JPMorgan Chase & Co. and Wells Fargo & Co. posted their earnings on Friday, other banks including Citigroup Inc., Goldman Sachs Group Inc., Bank of America Corp. and Morgan Stanley are on tap this week.

With volatility falling early in the year, Wall Street’s trading desks have faced a challenging environment and fewer chances to cash in on large price swings.

For example, JPMorgan’s revenue fell 17 percent in the first quarter from a year earlier, though it still exceeded overall profit and sales targets.

Health-care companies UnitedHealth Group Inc. and Johnson & Johnson are among the other big names on deck to report earnings, in addition to streaming-service giant Netflix, Inc.

Netflix shares have rallied more than 30 percent so far this year as technology and Internet stocks have driven major indexes higher, pushing the S&P 500 up 16 percent for the year and 0.8 percent below its September record. The index has climbed in 13 of the last 16 weeks.

However, many of the market’s most popular names have soared in 2019.

Total volume has been lower than the year-to-date average in seven consecutive sessions.

The VIX, which measures expected swings in the S&P 500, has tumbled 53 percent this year to its lowest level in six months.