Unit 5: Consumption and Investment Function

I. Orientation: The Macroeconomic Framework

Consumption and investment are the two major components of private domestic expenditure. In Keynesian macroeconomics, especially following John Maynard Keynes’s The General Theory (1936), current income influences consumption, while expected profitability and the cost of finance influence investment. Together, they affect aggregate demand, national income, employment, and economic growth.

• Central relationship: Aggregate expenditure includes consumption and investment, alongside government spending and net exports.
  • In a simple closed economy: (Y = C + I)
  • (Y) = national income or output, (C) = consumption expenditure, and (I) = investment expenditure.
• Flow perspective: Consumption is expenditure on currently produced goods and services; investment is expenditure that adds to productive capacity or inventories.
• Short-run assumption: Prices, technology, and productive capacity may be treated as fixed, so changes in spending primarily affect output and employment.
• Long-run perspective: Investment expands capital stock, while consumption supports demand for the goods and services produced.
• Key distinction: Consumption satisfies present wants; investment sacrifices some present consumption to increase future production and income.

II. Consumption Function — Income and Spending Relationship

A. Introduction to consumption function

The consumption function expresses the systematic relationship between disposable income and consumption expenditure. It shows how much households plan to consume at different income levels, assuming other influences remain unchanged.

• Basic Keynesian equation: The linear consumption function is written as:

C = a + bYd

• (C) = total consumption expenditure.
• (a) = autonomous consumption, which occurs even when disposable income is zero.
• (b) = marginal propensity to consume.
• (Y_d) = disposable income, equal to income after taxes and transfers.
  • Autonomous consumption: Households may finance essential spending through savings, borrowing, or assistance. Thus, (a) is positive even when (Y_d = 0).
  • Induced consumption: The term (bY_d) changes as disposable income changes; higher income generally produces higher consumption.
  • Average propensity to consume: The proportion of income consumed is:

APC = C / Yd

• APC = average propensity to consume; (C) = consumption; (Y_d) = disposable income.
  • Marginal propensity to consume: The additional consumption caused by an additional unit of income is:

MPC = ΔC / ΔYd

• (ΔC) = change in consumption; (ΔY_d) = change in disposable income.
• Normally, (0 < MPC < 1), because households usually consume part and save part of additional income.
  • Consumption and saving identity: Disposable income is divided between consumption and saving:

Yd = C + S

• (S) = saving.
• Therefore, (MPC + MPS = 1), where MPS is the marginal propensity to save.

B. Determinants of consumption

Consumption depends on disposable income, but households also respond to wealth, expectations, interest rates, prices, and social conditions.

• Disposable income: Higher disposable income usually increases consumption. If income rises by 1,000 monetary units and (MPC = 0.8), consumption rises by 800 units.
• Wealth: Households with greater financial assets, land, housing, or pensions may consume more at the same current income. A rise in share prices can increase consumption through a wealth effect.
• Interest rates: Lower interest rates reduce the reward for saving and make borrowing cheaper, encouraging spending on houses, vehicles, and durable goods.
• Price level and inflation: A rise in prices reduces the real purchasing power of money balances. Expected inflation may instead encourage households to buy before prices rise further.
• Expectations: Expectations of secure employment and rising future income encourage present consumption; fear of unemployment encourages precautionary saving.
• Taxation and transfers: Higher direct taxes reduce disposable income, while welfare payments and tax rebates raise household purchasing power.
• Distribution of income: Lower-income households generally have a higher MPC because necessities absorb much of their income. A redistribution toward such households can raise total consumption.
• Consumer credit: Easier access to credit permits households to purchase durable goods before they have accumulated the necessary cash.
• Social and demographic factors: Age, family size, urbanization, customs, and lifestyle influence saving and spending patterns.
• Long-run consumption pattern: The short-run Keynesian function emphasizes current income, while life-cycle and permanent-income approaches stress expected lifetime resources and permanent income.

III. Investment — Capital Formation and Entrepreneurial Spending

A. Investment concepts and types

Investment is expenditure on newly produced capital goods, inventories, construction, or other assets that contribute to future production. Buying an existing share or second-hand machine is a financial transfer, not necessarily current macroeconomic investment.

• Gross investment: Total spending on new capital goods before allowing for depreciation.
• Depreciation: The wearing out, obsolescence, or accidental loss of existing capital during a period.
• Net investment: The addition to the capital stock is:

Net investment = Gross investment − Depreciation

• Positive net investment means productive capacity is expanding.
• Zero net investment means gross investment only replaces depreciated capital.
  • Fixed investment: Expenditure on machinery, factories, offices, transport equipment, and infrastructure. A firm purchasing a new production line is making fixed investment.
  • Inventory investment: Changes in stocks of raw materials, work in progress, and finished goods.
• Planned inventory investment is deliberately chosen.
• Unplanned inventory investment occurs when actual sales differ from expected sales.
  • Residential construction: In national-income accounting, newly built houses are treated as investment because they provide housing services over time.
  • Autonomous investment: Investment unrelated directly to current income, such as government-supported infrastructure or a technological innovation.
  • Induced investment: Investment responding to changes in income, demand, or sales. Firms expand capacity when existing plants operate near full utilization.
  • Private and public investment: Private firms invest for profit, whereas governments invest in roads, schools, hospitals, and public utilities.
  • Replacement and expansion investment: Replacement restores worn-out capital; expansion increases the quantity or capability of productive assets.

B. Determinants of investment

Investment is unstable because it depends heavily on expectations about future demand and profitability. The investment decision compares expected returns with the cost of acquiring funds.

• Expected rate of return: Firms invest when the anticipated return from a project exceeds its cost. Keynes called the expected profitability of capital the marginal efficiency of capital.
• Interest rate: The interest rate is the opportunity cost of borrowed funds and a benchmark for evaluating projects. Lower rates make more projects profitable.
• Expected demand: Rising sales and strong orders encourage firms to add machines and premises. Weak demand produces postponed or cancelled investment.
• Business expectations and confidence: Optimistic expectations raise investment; uncertainty about taxes, regulations, exchange rates, or political stability reduces it.
• Technological progress: New technology can make existing equipment obsolete and create profitable opportunities. The introduction of automated machinery may reduce unit costs.
• Capacity utilization: Firms with idle factories can meet higher demand without new capital. Firms operating close to full capacity are more likely to invest.
• Cost of capital goods: Prices of machinery, construction materials, energy, and labor affect the total project cost and expected profit.
• Corporate profits and retained earnings: Profitable firms can finance investment internally, reducing dependence on bank loans or capital markets.
• Credit conditions: Banks’ willingness to lend and firms’ access to bonds or equity influence investment, especially for small businesses.
• Taxes and government policy: Investment allowances, accelerated depreciation, subsidies, and stable regulations can encourage capital formation.
• Accelerator effect: Investment may respond to changes in output rather than merely its level. If firms require three units of capital for each unit of output, an increase in desired output can produce a larger proportional rise in investment.

IV. Role of Consumption and Investment in Economic Growth and Business Expansion — Demand and Capacity

A. Role of consumption and investment in economic growth and business expansion

Consumption and investment reinforce one another in a growing economy: consumption creates markets for firms, while investment increases the economy’s ability to produce.

• Consumption as aggregate demand: Household purchases of food, clothing, transport, and services generate revenue for businesses. In the simple expenditure model:

Y = C + I

• An autonomous increase in consumption or investment raises planned expenditure and can increase equilibrium income.
  • Multiplier effect: If (MPC = 0.75), the simple spending multiplier is:

k = 1 / (1 − MPC) = 1 / 0.25 = 4

• (k) = multiplier; MPC = marginal propensity to consume.
• An autonomous investment increase of 100 monetary units can ultimately raise equilibrium income by up to 400 units, assuming no taxes, imports, inflation, or capacity constraints.
  • Investment and productive capacity: New factories, machinery, research facilities, and infrastructure raise labor productivity and potential output. Investment also creates employment during construction and production.
  • Business expansion: Strong consumption increases sales and cash flow, motivating firms to expand premises, hire workers, and purchase equipment. Expansion investment then permits greater future sales.
  • Capital deepening: When workers receive more or better capital equipment, output per worker can rise. For example, computer-controlled machinery may increase production without a proportional increase in labor.
  • Innovation and competitiveness: Investment in research, software, training, and technology can lower costs, improve quality, and help firms compete in domestic and international markets.
  • Employment and income feedback: Investment directly creates jobs and indirectly generates supplier income. Those incomes support further consumption, creating a circular expansion.
  • Demand-led limitation: If consumption rises while productive capacity is already fully used, firms may respond with higher prices rather than higher real output.
  • Supply-led limitation: Investment without adequate demand may create excess capacity, unsold inventories, and financial losses.
  • Sustainable growth: Long-term growth requires productive investment, but consumption must remain sufficiently strong to provide markets. Excessive consumption financed by debt can weaken future demand, while excessive saving can produce insufficient demand in the short run.
  • Macroeconomic balance: Stable growth requires coordination between household spending, business investment, public policy, savings, and available productive resources. A balanced expansion avoids both demand deficiency and inflationary overheating.