A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
The accelerator principle states that investment depends on the rate of change of output, not the level. Growing economies need more capital; slowing economies disinvest even if output remains positive.
Use numerical examples: if firms maintain capital-to-output ratio of 2:1 and output grows 10% in year 1, they need 10% more capital. If growth slows to 5% in year 2, investment falls despite rising output.
From your study of GDP components you know that investment is one of the most volatile components of aggregate expenditure. The accelerator principle explains why: investment is not driven by the level of output but by the *change* in output. This distinction is counterintuitive at first but becomes clear once you think about what investment is actually for — it is the act of expanding the capital stock to meet demand. If demand is not growing, there is no need to add capital.
Here is the core logic. Suppose firms maintain a target capital-to-output ratio — say, $2 of capital for every $1 of annual output (a ratio of 2). If output is $100, the desired capital stock is $200. If output rises to $110, the desired capital stock rises to $220. Firms must invest $20 to close the gap. Notice: investment of $20 is 20% of the *change* in output ($10), not of the output level itself. In the next year, if output grows again to $125, desired capital rises to $250, requiring $30 more investment. Investment rose because the *rate of growth* of output rose. Now suppose growth slows: output rises from $125 to $130. Desired capital goes from $250 to $260 — firms still invest, but only $10. Output is still rising, but investment *fell* because the pace of growth decelerated. This is the accelerator in action.
The amplification effect is the macroeconomic punchline. Because investment responds to the *rate of change* of output, small fluctuations in the growth rate of GDP produce large swings in investment. A mere slowdown (not a recession) is enough to cause investment to fall sharply, which then reduces aggregate demand further, which can slow growth even more. This creates the self-reinforcing dynamic that makes investment the most volatile component of GDP and a key driver of business cycles. The accelerator is one explanation for why downturns can deepen quickly: as soon as growth decelerates, investment collapses, amplifying the slowdown into a potential recession.
The accelerator principle also works in reverse and has important caveats in practice. During a recovery, as output growth picks up, the desired capital stock expands rapidly, triggering a surge in investment that can overshoot. In reality, the accelerator is a tendency rather than a mechanical law: firms face financing constraints, adjustment costs, and uncertainty about whether demand growth is permanent or temporary. They therefore smooth their investment rather than instantly closing the gap to the desired capital stock. These frictions are captured in more elaborate flexible accelerator models, where the adjustment to desired capital stock is partial each period. But the basic insight — that investment is inherently tied to the second derivative of output, not the level — is one of the most important dynamics in macroeconomics.