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acceleratorthe relationship between the amount of net or INDUCED INVESTMENT (gross investment less REPLACEMENT INVESTMENT) and the rate of change of NATIONAL INCOME. A rapid rise in income and consumption spending will put pressure on existing capacity and encourage businesses to invest, not only to replace existing capital as it wears out but also to invest in new plant and equipment to meet the increase in demand. By way of simple illustration, let us suppose a business meets the existing demand for its product, utilizing 10 machines, one of which is replaced each year. If demand increases by 20%, it must invest in two new machines to accommodate that demand in addition to the one replacement machine.
Investment is thus, in part, a function of changes in the level of income: I = f(AY). A rise in induced investment, in turn, serves to reinforce the MULTIPLIER effect in increasing national income.
The combined effect of accelerator and multiplier forces working through an investment cycle has been offered as an explanation for changes in the level of economic activity associated with the BUSINESS CYCLE. Because the level of investment depends upon the rate of change of GNP, when GNP is rising rapidly then investment will be at a high level, as producers seek to add to their capacity (time t in Fig. 2). This high level of investment will add to AGGREGATE DEMAND and help to maintain a high level of GNP. However, as the rate of growth of GNP slows down from time t onward, businesses will no longer need to add as rapidly to capacity, and investment will decline towards replacement investment levels. This lower level of investment will reduce aggregate demand and contribute towards the eventual fall in GNP. Once GNP has persisted at a low level for some time, then machines will gradually wear out and businesses will need to replace some of these machines if they are to maintain sufficient production capacity to meet even the lower level of aggregate demand experienced. This increase in the level of investment at time t1 will increase aggregate demand and stimulate the growth of GNP Like FIXED INVESTMENT, investment in stock is also to some extent a function of the rate of change of income so that INVENTORY INVESTMENT is subject to similar accelerator effects.