Even more startling, they argue, is the main source of this aging: a sharp drop in entrepreneurial activity. They define entrepreneurship as the number of startups — new firms ranging from plumbing to biotechnology. From 1978 to 2011, startups fell from about 15 percent of all firms to 8 percent; the slide was gradual until the 2008-09 financial crisis, when it accelerated. By these numbers, the economy’s rejuvenation from below is weakening; though conspicuous, the Internet’s influence is exaggerated.

Other studies reach similar conclusions. Shrinking entrepreneurship is hurting job creation and productivity, write economists Ryan Decker and John Haltiwanger of the University of Maryland and Ron Jarmin and Javier Miranda of the Census Bureau in the Journal of Economic Perspectives.

Start with jobs. From 1980 to 2010, U.S. employment increased by an average of 1.4 million jobs annually, report the economists. Over the same period, employment gains by startups averaged 2.9 million annually. By this math, startups accounted for more than the total gain in U.S. employment.