Investment - Aggregate Demand

In economics, investment (I) is spending by firms (and the public sector) on capital goods — the man-made resources used to produce other goods and services. This includes machinery, equipment, factories, vehicles, computers and new buildings, as well as additions to stock (inventories). It is one of the four components of aggregate demand:

$AD = C + I + G + (X - M)$

Investment is a smaller share of AD than consumption (often around 15–20%), but it is the most volatile component, because it depends heavily on business confidence and expected profit, which swing sharply over the economic cycle.

Crucially, investment is NOT the same as the everyday meaning of the word. When people say they have 'invested in shares' or 'invested their savings', they mean buying financial assets — swapping money for a claim on future income. That is not investment in the economic sense. Economic investment means creating or buying real capital goods that add to the economy's ability to produce output in the future. Buying an existing share simply transfers ownership; it does not add a new machine or factory to the economy.

Why investment matters twice over. Investment is unusual because it works in two ways:

• In the short run it is a component of AD — a firm buying new machinery is spending now, adding to total demand.
• In the long run that new machinery raises the economy's productive capacity — it shifts LRAS (long-run aggregate supply) to the right.

That dual role — demand now, capacity later — is what makes investment so important, and it runs through the whole of this subtopic.

Capital goods wear out over time — machines break down, vehicles age, buildings deteriorate. This wearing out (and obsolescence) is called depreciation (or capital consumption). To understand what investment actually adds to the economy, we must separate two measures.

Gross investment is the total amount firms spend on capital goods in a period — every new machine, factory and vehicle, whether it replaces worn-out capital or adds to the stock.

Net investment is gross investment after deducting depreciation — the spending that is left over once worn-out capital has been replaced:

$\text{Net investment} = \text{Gross investment} - \text{Depreciation}$

Why the distinction matters. The capital stock is the total quantity of capital goods in the economy. It only grows if net investment is positive:

• If gross investment = depreciation, net investment is zero: firms are only replacing worn-out capital, so the capital stock stays the same.
• If gross investment > depreciation, net investment is positive: the capital stock grows, raising future productive capacity.
• If gross investment < depreciation, net investment is negative: capital is wearing out faster than it is replaced, so the capital stock shrinks.

This is why net investment, not gross, is the figure that matters for long-run growth. Only positive net investment adds to the capital stock and shifts LRAS to the right. A country can have high gross investment yet a stagnant capital stock if most of that spending is simply replacing old capital.

Measure	What it counts	Effect on capital stock
Gross investment	ALL spending on capital goods	Includes replacement + additions
Depreciation	Wearing out of existing capital	Reduces the stock
Net investment (gross − depreciation)	Spending BEYOND replacement	Positive → stock grows; zero → unchanged; negative → shrinks

Because investment is volatile, examiners want you to know what moves it. Learn these determinants and be able to trace each one through to a change in I (and hence AD).

1. The rate of interest / cost of borrowing. The interest rate is the cost of borrowing to finance investment (and the opportunity cost of using retained profits). Lower interest rates make more projects profitable — the expected return now exceeds the cheaper cost of borrowing — so investment rises. Higher interest rates make borrowing dearer and fewer projects worthwhile, so investment falls. This is the channel monetary policy works through.

2. Business confidence and expectations ('animal spirits'). Investment is spending now for an uncertain return later, so it depends heavily on how firms feel about the future. Keynes called this 'animal spirits'. Optimistic firms expecting rising demand and profit invest heavily; pessimistic firms expecting recession postpone or cancel projects, even if interest rates are low. This is the main reason investment is so volatile.

3. Demand and the accelerator effect. Firms invest to meet expected demand for their output. The accelerator effect says investment depends on the rate of change of demand/output: when demand is growing, firms add capacity (high investment); when demand growth slows — even if output is still rising — firms need less new capacity, so investment can fall sharply. This magnifies the volatility of investment.

4. Corporation tax. This is a tax on company profits. A cut in corporation tax leaves firms with more retained (after-tax) profit to reinvest and raises the after-tax return on projects, encouraging investment. A rise in corporation tax does the opposite.

5. Retained profits and access to finance. Much investment is funded from retained profit (profit kept back rather than paid out). Higher current profits, and easier access to finance (bank lending, share issues), make investment easier to fund. In a credit crunch, even confident, profitable firms may be unable to borrow, so investment falls.

6. Technological change. New technology creates new, profitable investment opportunities and makes existing capital obsolete, prompting firms to replace it. Rapid technological progress tends to raise investment (and can raise depreciation too, as old capital is scrapped sooner).

7. The influence of government and regulation. Governments affect investment through infrastructure spending, tax incentives and grants, subsidies, the regulatory environment and general economic stability. Supportive policy and political/economic stability encourage investment; heavy regulation, uncertainty or instability discourage it.

Determinant	Change	Effect on investment
Interest rate / cost of borrowing	falls	rises (more projects profitable)
Business confidence ('animal spirits')	improves	rises (firms expect higher returns)
Demand / the accelerator	growth speeds up	rises (more capacity needed)
Corporation tax	cut	rises (more retained profit, higher after-tax return)
Retained profits / access to finance	improve	rises (easier to fund)
Technological change	advances	rises (new opportunities, replaces obsolete capital)
Government / regulation	supportive & stable	rises (incentives, confidence)

The link between the interest rate and investment is important enough to draw. Firms rank potential projects by their expected rate of return. A project is worth undertaking only if its expected return is at least as high as the cost of borrowing (the interest rate). So as the interest rate falls, more projects clear that hurdle and planned investment rises — giving a downward-sloping investment demand curve against the interest rate.

In the diagram, a fall in the interest rate from r to r₁ moves the economy down the curve, raising planned investment from I to I₁ — cheaper borrowing makes previously marginal projects worthwhile. A rise in the interest rate does the reverse.

Movement along vs a shift. A change in the interest rate causes a movement along the investment demand curve. A change in any other determinant — business confidence, expected demand (the accelerator), corporation tax, retained profits, technology or government policy — shifts the whole curve: greater confidence or a tax cut shifts it right (more investment at every interest rate); a slump in confidence shifts it left. This mirrors the movement-vs-shift rule you already know from demand and AD.

Investment is the one component of AD that also affects the economy's supply side, so it appears on both sides of the macro model.

Short run — investment as a component of AD. When firms spend on capital goods, that spending adds to total demand now. A rise in investment raises I in AD = C + I + G + (X − M), shifting AD to the right — raising real output and (nearer full capacity) the price level. A fall in investment shifts AD left. Because investment is volatile, it is a major cause of short-run fluctuations in AD and hence of the economic cycle.

Long run — investment as a driver of LRAS / productive capacity. The capital goods bought today are used to produce output for years to come. Positive net investment adds to the capital stock, and more (and better) capital raises the economy's productive capacity — shifting LRAS (or the PPF) to the right. This is a key source of long-run economic growth and of rising productivity and international competitiveness.

Why the dual role matters for evaluation. This is a powerful evaluation point. A rise in investment is 'doubly' beneficial: it boosts demand in the short run and supply in the long run — so it can raise growth without the inflationary pressure that a purely demand-side boost (like a consumption boom) would create, because capacity rises alongside demand. Conversely, a collapse in investment is doubly damaging: it cuts AD now and weakens future growth. The type and quality of investment matter too — investment in productive technology and infrastructure raises capacity more than replacement investment that merely maintains it.