2.2.3 - Investment (I)

What Investment Means

In macroeconomics, investment means expenditure by firms on capital goods such as machinery, factory buildings, vehicles, and equipment used in production. It does not mean households buying shares or putting money into a bank account.

Investment appears in the aggregate demand equation as the I in AD = C + I + G + (X - M). It is especially important because it has a dual role in the economy. In the short run, investment spending adds directly to aggregate demand. In the long run, it increases the economy's capital stock, meaning the quantity of physical capital available for production. That is why investment matters for both demand-side performance now and potential growth later.

Gross and Net Investment

Not all investment spending creates a genuine increase in productive capacity. Some of it simply replaces capital that has worn out or become obsolete.

Gross investment is the headline figure before allowing for wear and tear on existing capital. Economists therefore also need to consider depreciation, which is the fall in the value of capital goods over time because of wear and tear.

Once depreciation is subtracted, we get the figure that really matters for the economy's capacity to produce.

This distinction is crucial. If a firm spends £100 million on new machinery but £80 million of existing machinery wears out during the year, gross investment is £100 million but net investment is only £20 million. Net investment is the change in the capital stock, so the true increase in the capital stock is £20 million, not £100 million. If depreciation is greater than gross investment, net investment is negative, the capital stock shrinks, and sustained long-run growth becomes much harder. The link also works backwards: if a firm's capital stock falls over the year, its net investment was negative, so depreciation must have been larger than its gross investment.

Why Investment Is Volatile

Investment decisions are difficult because firms spend money today based on returns that may only appear years later. That makes investment heavily dependent on expectations about future demand and profit.

The rate of economic growth is therefore a major influence. When the economy is growing strongly, firms tend to experience higher sales and profits. Higher profits provide retained earnings that can finance investment internally, and stronger demand gives firms confidence that extra capacity will be used profitably. During a recession, the opposite tends to happen. Demand weakens, spare capacity appears, and firms see little reason to expand when they cannot fully use what they already have. This creates a feedback loop in which weak growth discourages investment and lower investment then weakens future growth.

Another useful exam idea is the accelerator effect. Because investment is spending on additions to capital stock, a small expected change in final demand can cause a much larger percentage change in planned investment. If firms think sales will rise, they may need a sizeable increase in machinery, vehicles, or factory space to meet that extra demand. For example, suppose a firm needs 10 machines to meet demand and normally buys 1 machine a year to replace one that wears out. If it expects demand to rise by 10%, it needs 11 machines, so this year it buys 2: a 10% rise in demand doubles its investment. If demand then stops growing, it is back to buying only a replacement machine, so investment halves even though sales have not fallen. Investment therefore depends on the rate of change of demand, not just its level. The same logic works in reverse during downturns, which helps explain why investment is often more volatile than consumption.

Business expectations matter beyond current sales figures. Firms may postpone investment because of uncertainty about future regulation, policy, elections, commodity prices, or the general economic outlook. Keynes argued in The General Theory (1936) that, because the future is so uncertain, many investment decisions rest less on precise calculation than on confidence and instinct, which he called animal spirits.