SURPRISE: Negative interest rates might be backfiring.

Policy makers in Europe and Japan have turned to negative rates for the same reason—to stimulate their lackluster economies. Yet the results have left some economists scratching their heads. Instead of opening their wallets, many consumers and businesses are squirreling away more money.

When Ms. Hofmann heard the ECB was knocking rates below zero in June 2014, she considered it “madness” and promptly cut her spending, set aside more money and bought gold. “I now need to save more than before to have enough to retire,” says Ms. Hofmann, 54 years old.

Surely some smart central banker could point to just one example where negative interest rates have produced the intended growth.