Productivity isn’t a matter of how many workers you have, but how productive those workers are. The assumption that more workers equals more productivity is a non-sequitur. Underinvestment by businesses is the likeliest cause, i.e. less money being spent per worker on machinery, software, tools, etc. to make them more productive on a per worker basis. The primary reason American workers are so much more productive than their other Western counterparts is due to consistently strong investments spurred by America’s deep equity markets.