Great achievements in knowledge are produced by older innovators today than they were a century ago. Nobel Prize winners and great inventors have become especially unproductive at younger ages. Meanwhile, the early life cycle decline is not offset by increased productivity beyond middle age. The early life cycle dynamics are closely related to age when the PhD was received, and I discuss a theory where knowledge accumulation across generations leads innovators to seek more education over time. More generally, the narrowing innovative life cycle reduces, other things equal, aggregate creative output. This productivity drop is particularly acute if innovators' raw ability is greatest when young.

This paper investigates the remarkable extremes of growth experiences within countries and the changes that occur across growth transitions. We find two main results. First, virtually all but the very richest countries experience both growth miracles and failures over substantial periods. Second, growth accelerations and collapses are asymmetric phenomena. Collapses typically feature reduced investment amidst increasing price instability, whereas growth takeoffs are primarily associated with large expansions in international trade. The results show that even very poor countries regularly grow rapidly, but sustaining growth is difficult and may pose a very different set of challenges than starting it. Copyright by the President and Fellows of Harvard College and the Massachusetts Institute of Technology.