Recognizing employees who do well improves workers’ performance overall—not because the recognized employees work harder, but because those who weren’t recognized do.

That was one of the conclusions of an academic study described in a 2013 working paper from European research center CESifo. The researchers assigned groups of students to three hours of data entry. During the shift, some of the highest performers received a thank-you card for their hard work from their manager. That sparked a jump in the groups’ productivity...