Back when the economy was dragging along like a narcoleptic turtle, there was talk that Americans simply needed better skills. CEOs complained it was difficult to find qualified hires, and the Obama Administration put an emphasis on retraining programs. But this “skills gap” was, at least in part, an artifact of the sluggish labor market, according to a new academic paper.

Three economists—from Northeastern University, Harvard and the Federal Reserve Bank of Boston—examined data for 36 million online job postings between...