Aggregate Demand and Its Components - Investment

In macroeconomics, investment has a specific, narrower meaning. It refers to the addition to the stock of physical capital (machines, buildings, roads, factories) and changes in the inventory (stock of finished goods) of a producer. This is called real investment or economic investment. Investment goods like machines are part of final goods — they are not "used up" in one production cycle but yield services over many years.​

• "Investment refers to the increment of capital equipment." — J.M. Keynes
• "By investment we do not mean the purchase of existing paper securities, bonds, debentures or equities, but the purchase of new factories, machines and the like". — Stonier and Hague
• "Investment expenditure includes expenditure for producer’s durable equipment, new construction and the change in inventories." — Peterson

The relationship between investment and the rate of interest can be written as:

I = f(r)

Here:

• I = Investment, the planned amount of investment; it is the dependent variable.​
• r = Rate of interest; it is the independent variable that influences investment.​

This notation means that the level of investment depends on the rate of interest.

P_I = `I / Y`
PI = Propensity to invest, I = Aggregate Investment, Y = Aggregate Income

Autonomous investment is expenditure on capital formation that does not depend on the level of income or profit in the economy. Its key features:

• Income-inelastic — remains the same whether the economy is booming or in recession​
• Primarily undertaken by the government for welfare and infrastructure​
• Driven by factors like new technology, population growth, or long-term policy goals — not by profit motive​
• Graphically represented as a horizontal straight line parallel to the X-axis

Real-life example: The Indian government's investment in building the Delhi-Mumbai Expressway continues regardless of whether GDP growth is 4% or 8%. Similarly, construction of government schools, rural roads under PMGSY, and public hospitals are autonomous investments — they happen for welfare, not profit.

Induced investment is expenditure on fixed assets and stocks that changes with the level of income and demand in the economy. Its key features:

• Income-elastic — as national income rises, induced investment rises; as income falls, it falls​
• Undertaken by private firms to earn profits​
• Driven by profit expectations and rising consumer demand
• Graphically represented as an upward-sloping line from left to right​
• Falls sharply during economic depression because profit expectations are low

Real-life example: When Indians' incomes rose post-COVID, demand for smartphones surged. Seeing this, companies like Foxconn and Samsung invested billions in new manufacturing plants in India. If demand falls, they would delay expansion. This is induced investment — it responds to income changes.

1) Gross Investment (Ig): Total expenditure on new capital goods plus replacement of worn-out capital in a given period.

2) Net Investment (In): The portion of gross investment that actually adds to the capital stock (after subtracting depreciation).

3) Replacement Investment (Ir): Expenditure on replacing depreciated or worn-out capital goods. Also called depreciation.

Key Formulas

Ig=In+Ir​
In=Ig−Ir​

Numerical Example

Important Rules:

• When Ig > Ir → Net investment is positive → Capital accumulation (economy is growing)
• When Ig = Ir → Net investment is zero → Economy is merely maintaining existing capital
• When Ig < Ir → Net investment is negative → Economy's capital stock is shrinking (decay)

Ex-ante means "before the event." Ex-ante investment is the amount of investment that firms plan or intend to make during a given period. It is based on:

• Expected future demand and profit opportunities
• Desire to reduce production costs by adopting new technology
• Government targets for employment or economic growth​

Example: At the start of the financial year, Tata Steel plans to invest ₹500 crore in a new blast furnace. This is an ex-ante investment.

Ex-post means "after the event." Ex-post investment is the actual investment that takes place, including unintended changes in inventory.

Example: A shoe manufacturer produces 10,000 pairs, expecting to sell all of them. Due to a sudden drop in demand, only 7,000 sell. The 3,000 unsold pairs are unplanned inventory investment — the firm didn't want this stock, but it counts as investment.​

Average Propensity to Invest (API)

The ratio between total investment and total income:

\[API=\frac{I}{Y}\]

Marginal Propensity to Invest (MPI)

The ratio of change in investment to change in income:

\[MPI=\frac{\Delta I}{\Delta Y}\]

Example: If national income rises from ₹1,000 Cr to ₹1,200 Cr and investment rises from ₹200 Cr to ₹230 Cr, then MPI = 30/200 = 0.15. This means for every additional ₹1 of income, ₹0.15 goes into investment.

• Economic investment = addition to physical capital + change in inventories — NOT buying shares/bonds​
• Autonomous investment is income-inelastic, welfare-driven, mostly by government; drawn as a horizontal line​
• Induced investment is income-elastic, profit-driven, mostly private; drawn as an upward-sloping line​
• Gross Investment = Net Investment + Depreciation; net investment positive means capital accumulation
• Ex-ante = planned; Ex-post = actual; equilibrium requires ex-ante S = ex-ante I​
• Investment function I = f(r) is downward-sloping — higher interest means less investment​
• Invest when MEI > Rate of Interest; stop when MEI = Rate of Interest​