This comment from Chris is interesting:
I would speculate that the the massive productivity gains were due to a massive resorting of American society along cognitive lines; from 1940 to 1970 a large number of high ability people who were previously locked into agriculture and industry were able to sort themselves into more innovative positions. This would lead to a massive burst of innovation, which led to increases in productivity, as previously unlocked talent was put to use. From 1970 to 1990 this resorting was mostly winding down and productivity in the economy was heavily constrained by the population due to people still being necessary for most tasks and most of the potential innovation overhang being used up from 1940 to 1970. Although computers were increasingly able to challenge people on some tasks during the 1970's, their effect was small (although increasingly noticeable in how long it took employment to ...
