The accelerator effect in economics is a positive effect on private fixed investment of the growth of the market economy (measured e.g. by a change in Gross Domestic Product). Rising GDP (an economic boom or prosperity) implies that businesses in general see rising profits, increased sales and cash flow, and greater use of existing capacity. This usually implies that profit expectations and business confidence rise, encouraging businesses to build more factories and other buildings and to install more machinery. (This expenditure is called fixed investment.) This may lead to further growth of the economy through the stimulation of consumer incomes and purchases, i.e., via the multiplier effect.

Every firm has some strategies to work which usually make the progress towards achieving an optimum capital stock and not only moving smoothly from one type and size of plant and machinery to the other. This means that every firm aims to increase its profit to an optimum level rather than just moving and improving its machinery and buildings. The accelerator theory concept was mainly given by Thomas Nixon Carver and Albert Aftalion before Keynesian economics came into force but it came into public knowledge more and more as the Keynesian theory began to dominate the field of economics. Some people criticized it and also argued against the accelerator theory because it was thought to remove all the possibility of the demand control through the price control mechanism.

The accelerator effect also goes the other way: falling GDP (a recession) hurts business profits, sales, cash flow, use of capacity and expectations. This in turn discourages fixed investment, worsening a recession by the multiplier effect.

The accelerator effect fits the behavior of an economy best when either the economy is moving away from full employment or when it is already below that level of production. This is because high levels of aggregate demand hit against the limits set by the existing labour force, the existing stock of capital goods, the availability of natural resources, and the technical ability of an economy to convert inputs into products.