The marginal efficiency of capital (MEC) is that rate of discount which would equate the price of a fixed capital asset with its present discounted value of expected income.
The term “marginal efficiency of capital” was introduced by John Maynard Keynes in his General Theory, and defined as “the rate of discount which would make the present value of the series of annuities given by the returns expected from the capital asset during its life just equal its supply price”.
The General Theory of Employment, Interest and Money is a book by English economist John Maynard Keynes published in February 1936. It caused a profound shift in economic thought, giving macroeconomics a central place in economic theory and contributing much of its terminology – the "Keynesian Revolution". It had equally powerful consequences in economic policy, being interpreted as providing theoretical support for government spending in general, and for budgetary deficits, monetary intervention and counter-cyclical policies in particular. It is pervaded with an air of mistrust for the rationality of free-market decision-making.
Keynes denied that an economy would automatically adapt to provide full employment even in equilibrium, and believed that the volatile and ungovernable psychology of markets would lead to periodic booms and crises. The General Theory is a sustained attack on the classical economics orthodoxy of its time. It introduced the concepts of the consumption function, the principle of effective demand and liquidity preference, and gave new prominence to the multiplier and the marginal efficiency of capital.