The tide has suddenly turned on the economics consensus among everyone from Keynesian professors to Wall Street commentators. Their expectations for a soft landing have fallen to earth.

The immediate trigger for the shift and the selloff in equity markets was a run of adverse data last week. It began on Wednesday, with higher claims for unemployment insurance, followed on Thursday by weak purchasing-manager indexes for manufacturing and services. Then on Friday came disappointing nonfarm payroll data and a higher than expected unemployment figure.

To explain why the consensus changed so fast, the economic chattering classes and press have latched onto the Sahm rule. That tool, created by economist Claudia Sahm, correlates an increase in unemployment with the onset of recessions. According to Ms. Sahm’s research, if the unemployment rate climbs by half a percentage point or more relative to its low during the previous 12 months, we will be in the early months of a recession.