Investors are elated by a booming global economy and the promise of central banks to tighten monetary policy only gradually. But a question haunts them: Will interest rates develop a mind of their own?

While central banks set short-term rates—the 1.5% rate that the Federal Reserve publishes on its website—economists disagree about how much control they have over long-term borrowing costs. These are gauged by government-bond yields, especially those with returns tied to inflation.

Low inflation-indexed—or "real"—rates push money into risky assets, because investors get little extra purchasing power for holding safer securities. According to a new report by BlackRock Inc., the world's biggest asset manager, subdued real rates have been 2017's main driver of returns in global infrastructure debt and investment-grade corporate debt. They also boost gold and real estate, analysts say, which don't pay coupons but don't lose value when inflation rises.

Many markets could climb off record highs if real rates rise. But it is hard to forecast, said Kevin Gardiner, global investment strategist at Rothschild Wealth Management, because "nobody knows exactly what sets interest rates."

Real rates have often moved in lockstep with central-bank policy—but not always. In the 1970s, runaway inflation pushed real rates down even as the Fed and other central banks increased nominal rates.