A Decline in the Average Propensity

The savings rate and the average propensity to consume together sum to 1. In other words, a decline in the average propensity to consume equivalently means that households are saving a larger fraction of their income. Because the consumption and savings relationships are two sides of the same coin, economists wishing to find the actual values of autonomous consumption and the marginal propensity to consume can examine data on consumption, savings, or both. If the data were perfect, we would get the same answer either way. For the United States, both consumption and savings data are readily available, but in some countries the data on savings may be of higher quality than the consumption data, in which case economists use savings data to understand consumption behavior. Some Warnings about the Consumption Function The consumption function is useful because it captures two fundamental insights: households seek to smooth their consumption, but consumption nonetheless responds to current income. But the consumption function is really too simple. [4] First, it ignores the role of accumulated wealth. If you consider two households with the same level of current income but different amounts of accumulated wealth, the one with higher wealth will probably consume more. Second, the consumption function does not explicitly include the role of expectations. A household’s consumption reflects not only income today and the accumulation of income in the form of wealth but also anticipated income. So, for example, if a government announces that it will increase income tax rates in two years, we expect that households will respond immediately to smooth out the effects of these future taxes. The only way the consumption function allows us to capture wealth or expectations of future income is through autonomous consumption. This is fine as far as it goes, but it means that we are taking too many aspects of consumption as given, rather than explaining them with our theory. Another complication is that changes in income today are often correlated with changes in income in the future. If your income increases today, is this an indication that your income will also be higher in the future? To see why this matters, consider two extreme examples. First, suppose that you receive a one-time inheritance of $10 million. What will you do with this income? According to the consumption smoothing argument, you will save some of this income to increase your consumption in the future. Roughly speaking, if you thought you had 10 years left to live, you might increase your consumption by about $1 million per year. In this case your marginal propensity to consume would be only 0.1. Now suppose that instead of a $10 million windfall, you learn you will receive $1 million each year for the next 10 years. In this case, your income is already spread out over your lifetime. So, in this second case, you will again want to smooth your consumption. But since the increase in income will be maintained for your lifetime, you can increase your consumption by an amount equal to the increase in your income. Your marginal propensity to consume will be 1.0. The difference between these two situations is that in the first case the income increase is temporary, and in the second it is permanent. The logic of consumption smoothing implies that the marginal propensity to consume is near 1 for permanent changes in income but much smaller for temporary changes in income. The Effects of a Change in Income Taxes We can now figure out the effects of a cut in taxes on consumption and saving. A reduction in taxes will increase disposable income. From the consumption function, this results in an increase in consumption equal to the marginal propensity to consume times the increase in disposable income. The average propensity to consume decreases. To summarize, if taxes are cut in the economy, we expect to see the following:

 An increase in disposable income  An increase in consumption that is smaller than the increase in disposable income

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(that is, a marginal propensity to consume less than 1)  A decline in the average propensity to consume