Every time there is an injection of new demand into the circular flow of income there is likely to be a multiplier effect. This is because an injection of extra income leads to more spending, which creates more income, and so on. The multiplier effect refers to the increase in final income arising from any new injection of spending.

The size of the multiplier depends upon household’s marginal decisions to spend, called the marginal propensity to consume (mpc), or to save, called the marginal propensity to save (mps). It is important to remember that when income is spent, this spending becomes someone else’s income, and so on. Marginal propensities show the proportion of extra income allocated to particular activities, such as investment spending by UK firms, saving by households, and spending on imports from abroad. For example, if 80% of all new income in a given period of time is spent on UK products, the marginal propensity to consume would be 80/100, which is 0.8.

Source: ForumIAS