Capital in the Twenty-First Century

by Thomas Piketty

Recall that g measures the long-term structural growth rate, which is the sum of productivity growth and population growth. In Marx’s mind, as in the minds of all nineteenth- and early twentieth-century economists before Robert Solow did his work on growth in the 1950s, the very idea of structural growth, driven by permanent and durable growth of productivity, was not clearly identified or formulated.31 In those days, the implicit hypothesis was that growth of production, and especially of manufacturing output, was explained mainly by the accumulation of industrial capital. In other words, output increased solely because every worker was backed by more machinery and equipment and not because productivity as such (for a given quantity of labor and capital) increased. Today we know that long-term structural growth is possible only because of productivity growth. But this was not obvious in Marx’s time, owing to lack of historical perspective and good data.