CHAPTER

16

Consumption

Questions for Review
1. First, Keynes conjectured that the marginal propensity to consume—the amount consumed out of an additional dollar of income—is between zero and one. This means that if an individual’s income increases by a dollar, both consumption and saving increase. Second, Keynes conjectured that the ratio of consumption to income—called the average propensity to consume—falls as income rises. This implies that the rich save a higher proportion of their income than do the poor. Third, Keynes conjectured that income is the primary determinant of consumption. In particular, he believed that the interest rate does not have an important effect on consumption. A consumption function that satisfies these three conjectures is C = C + cY. C is a constant level of “autonomous consumption,” and Y is disposable income; c is the marginal propensity to consume, and is between zero and one. 2. The evidence that was consistent with Keynes’s conjectures came from studies of household data and short time-series. There were two observations from household data. First, households with higher income consumed more and saved more, implying that the marginal propensity to consume is between zero and one. Second, higher-income households saved a larger fraction of their income than lower-income households, implying that the average propensity to consume falls with income. There were three additional observations from short time-series. First, in years when aggregate income was low, both consumption and saving were low, implying that the marginal propensity to consume is between zero and one. Second, in years with low income, the ratio of consumption to income was high, implying that the average propensity to consume falls as income rises. Third, the correlation between income and consumption seemed so strong that no variables other than income