The U.S. economy added fewer jobs in January than economists had forecast, although the jobless rate edged lower.

Friday’s report provided evidence of slowing expansion. The 143,000 jobs added would be the weakest January total since 2016.

But first a bit of background: The numbers in Friday’s jobs report most likely will change in the months (and years) to come.

As the labor market cools, American workers are losing confidence in their ability to find better pay or working conditions by changing jobs. The number of people quitting has fallen from a record 4.5 ...