What it means
Start with a factory already running at full capacity. If demand rises, the firm must buy more machines to serve it, so new investment is driven by the increase in output rather than by the output itself.
The consequence is amplification. If output grows strongly one year and only modestly the next, investment can fall in absolute terms even though sales are still growing, which is why order books for capital goods collapse before any recession is visible in consumer figures.
It is applied through the capital-output ratio, the amount of capital needed to produce one dollar of annual output. Multiply that ratio by the change in output and you get the induced investment the principle predicts.
The principle holds most tightly when an industry is already at full capacity, because spare capacity lets firms meet extra demand without buying anything at all. That is the main reason actual investment is far less mechanical than the simple version suggests.
The usual refinement is the flexible accelerator, which assumes firms close only part of the gap between desired and actual capital each year. It produces smoother and more realistic investment paths while keeping the central insight that changes in demand, not levels, drive capital spending.
The principle was first set out in the early twentieth century to explain why capital goods industries boomed and slumped harder than the rest of the economy. It remains the standard explanation for why a gentle slowdown in consumer demand can still feel like a crash to the firms that supply machinery, vehicles and buildings.
In practice
Real-world examples.
Example
A machine tool maker sees orders double when car plants expand, then fall by half the next year once the plants finish their expansion. Nothing has gone wrong with car sales, and the swing is the acceleration principle at work.
Example
A commercial property developer tracks office employment growth rather than total employment, because new building is only needed when headcount is rising. When growth flattens, its pipeline empties within two quarters.
Example
A steel distributor plans its stock around the change in construction starts rather than their level. By watching how fast starts are changing rather than how many there are, it avoids holding $4,000,000 of stock into a slowdown. The distributor also warns its board every January that a good year for builders can still be a poor year for the people who supply them.
Formula
Calculation
Induced investment = capital-output ratio x change in output. Suppose an industry needs $2 of capital for every $1 of annual output, so the capital-output ratio is 2. Output rises from $50,000,000 to $60,000,000, a change of $10,000,000, so induced investment is 2 x $10,000,000 = $20,000,000. The following year output rises again, but only from $60,000,000 to $65,000,000, a change of $5,000,000, so induced investment is 2 x $5,000,000 = $10,000,000. Demand is still growing, yet capital spending has halved, and if output simply held at $65,000,000 the induced investment would be 2 x $0 = $0 even though the industry is at a record size.Case study
Seen in the real world.
Velloway Presses is an illustrative and fictional maker of industrial printing presses selling mainly to packaging plants. When packaging output grew 12% in one year, Velloway's order book grew 40%, and the founder hired 60 extra staff to cope.
The next year packaging output still grew, but by only 4%, and Velloway's orders fell by a third. The business was profitable, its customers were healthy and its product was fine, yet it had to cut the workforce it had just built.
After that fictional cycle, Velloway began reporting a rolling forecast of its customers' capacity utilisation alongside its own order book, and used contract labour for the top 20% of capacity. The illustrative point is that suppliers of capital goods must plan around the change in their customers' growth rate, not their customers' size.
Watch out
Common mistakes.
- Reading falling investment as proof that demand is falling. Investment can drop sharply while demand is still rising, simply because it is rising more slowly than before.
- Applying the principle to an industry with spare capacity. Firms with idle machines can serve extra demand without investing at all, which breaks the link completely.
- Forgetting replacement spending. Even with no growth, firms keep buying equipment to replace worn-out assets, so total investment never actually falls to zero.
Questions
People also ask.
Is the acceleration principle the same as the multiplier?
No, the multiplier describes how spending ripples through incomes, while the acceleration principle describes how changes in output drive investment.
What is the capital-output ratio in practice?
It is capital employed divided by annual output, and it varies widely by sector, from well under one in services to several times output in heavy industry.
Why should a non-economist care about it?
Because it tells any business selling equipment, materials or construction services that its revenue swings will be larger than its customers', and that it should plan capacity and hiring accordingly.
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