In a field experiment with more than 300 workers on a data-entry shift, unannounced thank-you cards were handed out partway through. When everyone got a card, output rose about 5%. When only the top three got one, output rose about 7% — and the non-recipients improved too.
Specific recognition does two jobs: it rewards the person, and it shows everyone else what good looks like. Thanking everyone equally does the first job weakly and the second not at all.