I was a Personnel Officer (Kayser Bondar & Yardleys) during the 1950s & a member of the Institute of Personnel Management. The above American conclusion was then accepted in that as a result of long hours in factories during the war, surveys had concluded that the rate of production did not increase - in fact the rate of production slowed & accidents increased. I would suspect that Factory Inspectors' reports made reference to that conclusion.
This knowledge appears to have been overlooked in recent years.
