Investment Theory in Macroeconomics

Investment Theory is a core component of macroeconomics. Its core research focus is the decision-making mechanisms and influencing factors of investment by enterprises and residents, as well as the interaction between investment and national income, economic cycles, and economic growth. In macroeconomics, investment is a vital part of aggregate demand (calculated by the expenditure approach to GDP: $GDP = C+I+G+NX$). Meanwhile, it serves as the source of capital accumulation. Its fluctuations are a key factor triggering economic cycles, and long-term investment growth is the core driving force for economic expansion.

Investment in macroeconomics mainly refers to real investment, i.e., the purchase and accumulation of physical capital, including corporate fixed asset investment (factories and equipment), inventory investment, and residential housing investment. It is distinct from financial investment such as securities trading in financial markets.

I. Keynes' Investment Determination Theory

Keynes' investment theory is the foundation of macro investment analysis, which core emphasizes that investment is determined by the comparison between the marginal efficiency of capital and the market interest rate.

1. Core Concepts

1. Marginal Efficiency of Capital (MEC)
   The marginal efficiency of capital refers to the discount rate at which the present value of the expected returns from a capital asset over its service life equals the supply price (acquisition cost) of that asset.
   Essentially, it represents the expected rate of profit of an investment project and reflects enterprises' expectations of the profitability of investment. Keynes argued that the marginal efficiency of capital has a diminishing trend: as the scale of investment expands, on the one hand, the price of capital goods will rise, increasing investment costs; on the other hand, the increase in product supply will lower product prices, reducing the expected returns of investment. Ultimately, this leads to a decline in the marginal efficiency of capital.

2. Market Interest Rate (r)
   The market interest rate is the opportunity cost of investment. Enterprises' investment funds either come from their own capital (forgoing deposit interest) or borrowed funds (paying loan interest). The higher the interest rate, the greater the opportunity cost of investment.

2. Core Criterion for Investment Decision-Making

The prerequisite for enterprises to conduct investment is: Marginal Efficiency of Capital ≥ Market Interest Rate ($MEC \geq r$).

• When $MEC > r$, the expected rate of profit of the investment project is higher than the opportunity cost, and enterprises will increase investment;
• When $MEC < r$, the investment project will result in losses, and enterprises will reduce investment;
• When $MEC = r$, the investment scale reaches an equilibrium level.

From this, the investment function can be derived: $I = I_0 - dr$
In the formula, $I_0$ is autonomous investment (investment not affected by interest rates, such as the necessary investment to maintain the existing production scale), $d$ is the sensitivity coefficient of investment to interest rates, and $r$ is the market interest rate. This function indicates that investment has an inverse relationship with interest rates.

3. The Key Role of Expectations

Keynes particularly emphasized the impact of entrepreneurs' expectations on investment. The marginal efficiency of capital is a value based on future expectations. If entrepreneurs are optimistic about the future economic outlook, they will raise the expected returns of investment, increase the marginal efficiency of capital, and thus expand investment; if they are pessimistic about the future, they will lower the marginal efficiency of capital and cut investment. The instability of expectations is also an important reason for the sharp fluctuations in investment.

II. Tobin's q Theory

Tobin's q Theory connects financial market prices with real economic investment, providing a new perspective for analyzing investment decisions. Its core is to judge whether enterprises should invest through the "q value".

1. Definition of q Value

Tobin defined the q value as the ratio of the market value of an enterprise to the replacement cost of its assets, with the formula:
$$q = \frac{\text{Market Value of the Enterprise}}{\text{Replacement Cost of the Enterprise's Assets}}$$

• Market Value of the Enterprise: Refers to the total market value of the enterprise's stocks, equal to the stock price per share multiplied by the number of common shares outstanding;
• Replacement Cost of the Enterprise's Assets: Refers to the cost required to repurchase all existing assets of the enterprise.

2. Relationship Between q Value and Investment Decision-Making

1. When $q > 1$: The market value of the enterprise is higher than the replacement cost of its assets. At this time, the enterprise can raise funds by issuing stocks and use the raised capital to purchase new equipment and build new factories. The cost is lower than directly acquiring existing enterprises, so enterprises will increase investment.
2. When $q < 1$: The market value of the enterprise is lower than the replacement cost of its assets. At this time, the cost of establishing a new enterprise is higher than acquiring existing ones. Enterprises prefer to acquire other enterprises rather than make new investments, so investment will decrease.
3. When $q = 1$: The market value of the enterprise is equal to the replacement cost of its assets, and investment reaches an equilibrium state.

The significance of Tobin's q Theory is that it reveals that monetary policy can affect investment by influencing stock prices. For example, when the central bank lowers interest rates, it will drive up stock prices, increase the q value, and stimulate corporate investment.

III. Accelerator Model (Core Part of the Multiplier-Accelerator Model)

The Accelerator Model explains investment fluctuations from the perspective of output changes. Its core view is that investment is not determined by the level of output, but by the rate of change of output. The fluctuation range of investment will be larger than that of output, a phenomenon known as the acceleration principle.

1. Core Concept: Accelerator

The accelerator ($v$) refers to the capital-output ratio, i.e., the amount of capital stock required to produce one unit of output, with the formula:
$$v = \frac{K}{Y}$$
In the formula, $K$ is the capital stock and $Y$ is the output level. Assuming the capital-output ratio is fixed (unchanged technical level), the relationship between the change in capital stock ($\Delta K$) and the change in output ($\Delta Y$) is:
$$\Delta K = v \cdot \Delta Y$$
Since net investment ($I_n$) is equal to the change in capital stock ($\Delta K$), the formula for net investment is:
$$I_n = v \cdot \Delta Y$$
Gross investment ($I$) = Net investment ($I_n$) + Depreciation ($D$), i.e.,
$$I = v \cdot \Delta Y + D$$

2. Core Conclusions of the Acceleration Principle

1. The change in investment depends on the rate of change of output, not the level of output. Only when output grows will investment increase; if the growth rate of output slows down (even if output is still growing), investment will decrease.
2. The fluctuation range of investment is much larger than that of output. Because the accelerator $v$ is usually greater than 1, a small change in output will trigger a large change in investment, which is why investment has become the main driving force of economic cycle fluctuations.

3. Interaction Between Multiplier and Accelerator

The Accelerator Model is often combined with Keynes' multiplier effect to form the Multiplier-Accelerator Model, which is used to explain the formation mechanism of economic cycles:

• Output growth → Investment increases at an accelerated pace (accelerator effect) → Aggregate demand expands → Output grows further (multiplier effect) → The economy enters an expansionary phase;
• The growth rate of output slows down → Investment decreases at an accelerated pace → Aggregate demand contracts → Output declines further → The economy enters a contractionary phase.

IV. Other Factors Affecting Investment

In addition to interest rates, expectations, output changes, and stock prices, investment is also affected by the following factors:

1. Tax Policies: The government can reduce corporate investment costs, increase the expected returns of investment, and stimulate investment through tax incentives (such as investment tax credits and corporate income tax reductions/exemptions). Conversely, raising tax rates will curb investment.
2. Financing Constraints: Enterprise investment requires capital support. If credit in the financial market tightens and small and medium-sized enterprises have difficulty obtaining loans (strong financing constraints), they cannot expand investment even if investment projects are highly profitable.
3. Technological Progress: The emergence of new technologies and equipment will improve production efficiency, create new investment opportunities, and drive enterprises to carry out renovation and upgrading investment.
4. Uncertainty: Uncertainties in economic policies, market demand, and raw material prices will increase investment risks for enterprises, leading them to postpone or reduce investment.

V. The Relationship Between Investment and the Macroeconomy

1. Short-term and Economic Cycles: Investment is the most volatile component of aggregate demand. Sharp fluctuations in investment will directly trigger fluctuations in aggregate demand, leading to the alternation of economic prosperity and recession, making it the "engine" of economic cycles.
2. Long-term and Economic Growth: Investment forms capital stock. In the long run, capital accumulation is an important factor driving economic growth (along with labor and technological progress). According to the Neoclassical Growth Theory, the per capita capital stock in the steady state depends on the savings rate (investment rate). Appropriately increasing the investment rate helps raise the level of long-term per capita output.