Stock-market investors, already grappling with the tech rout and the threat of a trade war, are starting to reassess a fundamental premise of the powerful rally—that world-wide economic growth is on the verge of blasting out of a long period of weakness.

The world’s major economies started to pick up steam together last year, in a break from years of sluggish post-crisis growth in which the U.S. often seemed like the lone bright spot. Global output expanded by 3.7% in 2017, up half a point from 2016, according to International...