Capital in the Twenty-First Century

by Thomas Piketty

In the standard economic model, based on the existence of a “perfect” market for capital (in which each owner of capital receives a return equal to the highest marginal productivity available in the economy, and everyone can borrow as much as he or she wants at that rate), the reason why the return on capital, r, is systematically and necessarily higher than the growth rate, g, is the following. If r were less than g, economic agents, realizing that their future income (and that of their descendants) will rise faster than the rate at which they can borrow, will feel infinitely wealthy and will therefore wish to borrow without limit in order to consume immediately (until r rises above g). In this extreme form, the mechanism is not entirely plausible, but it shows that r > g is true in the most standard of economic models and is even more likely to be true as capital markets become more efficient.