Keynesian Investment Theory: MEC and Animal Spirits

Posted on May 17, 2026 in Business Administration and Management (BAM)

Keynesian Theory of Investment

John Maynard Keynes’s theory of investment is a central component of his General Theory, offering a psychological and economic explanation for how businesses decide to purchase new capital goods, such as machinery, factories, and buildings.

Keynes argued that the level of investment in an economy is highly volatile and is determined by two main factors:

  1. The Marginal Efficiency of Capital (MEC): The expected rate of return on a new investment project.
  2. The Market Rate of Interest: The cost of borrowing funds to finance the investment.

The Marginal Efficiency of Capital (MEC)

The MEC is the expected profitability of undertaking an additional unit of investment. It is not a fixed number but is estimated by business owners based on their expectations of future revenue generated by the capital asset.

Keynes defined the MEC as the rate of discount that makes the present value of the prospective future returns from a capital asset equal to its supply price (or replacement cost):

Supply Price = R1/(1+k) + R2/(1+k)² + … + Rn/(1+k)ⁿ

Where:

  • R1, R2, … Rn are the expected returns in years 1, 2, … n.
  • k is the Marginal Efficiency of Capital (MEC).

Firms rank potential investment projects from the highest MEC to the lowest, creating an MEC schedule or demand curve for investment, which slopes downwards as the best projects are undertaken first.

Animal Spirits and Expectations

A crucial aspect of Keynes’s theory is the role of expectations. The prospective returns (Rn) are often based on uncertain, long-term forecasts. Keynes introduced the term “animal spirits” to describe the volatile, subjective confidence and optimism of entrepreneurs.

When business confidence is high (strong animal spirits), the MEC schedule shifts outward, leading to more investment. When confidence is low, the MEC schedule shifts inward, causing investment to dry up, even if the interest rate is low. This psychological volatility makes investment the most unstable component of aggregate demand.

The Rate of Interest

The interest rate represents the explicit or implicit cost of financing the capital project. It is the opportunity cost of holding the funds required for investment:

  • If a firm borrows money, the interest rate is the loan repayment cost.
  • If a firm uses its own savings, the interest rate is the return it forgoes by not lending that money out.

The Investment Decision

A rational firm applies a simple decision rule based on comparing the expected return (MEC) with the cost of capital (interest rate, i):

  • If MEC > i: The project is profitable and the firm will invest.
  • If MEC < i: The project is unprofitable and the firm will not invest.
  • If MEC = i: The firm is at the optimal level of investment.

The Aggregate Investment Demand Curve is derived by summing all profitable projects at various interest rates. As the interest rate falls, more projects become viable, and investment increases.

Policy Implications

Keynes’s theory of investment has major policy implications:

  • Monetary Policy: Central banks can lower interest rates to stimulate investment by making more projects profitable.
  • Fiscal Policy: Government spending can boost overall demand, improving expectations of future returns (Rn), which increases the MEC itself.
  • Instability: The emphasis on volatile “animal spirits” explains why market economies can experience booms and busts that governments may need to stabilize.