Investment is one of the most important variables in economics. On its back, humans have ridden from caves to skyscrapers. Its surges and collapses are still a primary cause of recessions. Indeed, as can be seen in Figure 1, investment has dropped sharply during almost every postwar U.S. recession. As the graph suggests, one cannot begin to project where the economy is going in the near term or the long term without having a firm grasp of the future...