To complete my discussion, I want to turn briefly to the question of why periods of strong trend productivity growth come to an end.
Several hypotheses have been put forth, including (1) that successful new technologies eventually lead to financial imbalances and overinvestment associated with excess optimism, (2) that periods of strong productivity growth eventually run out of steam as the  51  There is considerable debate on this issue. See Lipsey (2000) for a brief summary.