Amazon Is Taking Over the Stock Market, Too

Three technology titans have powered nearly half of the S&P 500’s advance this year, a worrying sign for investors expecting a strengthening economy to lift shares of manufacturers, oil companies and other firms whose fortunes typically improve with growth.

Amazon.com Inc. has accounted for 27% of the broader index’s 1.6% gain through Tuesday, according to S&P Dow Jones Indices’ data. That is followed by Microsoft Corp., which has contributed 13%, and Netflix Inc. at 8.3%. U.S. stocks fell Wednesday, but Amazon and...