Ultimately, the real yield on stocks - and, therefore, through arbitrage, the real yield on longer bonds - is determined by the profitability of corporations and the real return that they earn on their capital. It is useful to distinguish between physical and financial returns. The return on physical capital includes here the return on the organization, management, and the so-called human capital - that is, the education, training, experience, and esprit de corps of the workforce.
— Applied Intermediate Macroeconomics
by Kevin D. Hoover