In their studies of consumption, economists generally draw upon a common theoretical framework by assuming that consumers base their expenditures on a rational and informed assessment of their current and future economic circumstances. This “rational optimization” assumption is untestable, however, without additional assumptions about why and how consumers care about their level of consumption; therefore consumers’ preferences are assumed to be captured by a utility function. For example, economists usually assume (1) that the urgency of consumption needs will decline as the level of consumption increases (this is known as a declining marginal utility of consumption), (2) that people prefer to face less rather than more risk in their consumption (people are risk-averse), and (3) that unavoidable uncertainty in future income generates some degree of precautionary saving. In the interest of simplicity, the standard versions of these models also make some less-innocuous assumptions, including assertions that the pleasure yielded by today’s consumption does not depend upon on one’s past consumption (there are no habits from the past that influence today’s consumption) and that current pleasure does not depend upon comparison of one’s consumption to the consumption of others (there is no “envy”).

Within the rational optimization framework, there are two main approaches. The “life-cycle” model, first articulated in Utility Analysis and the Consumption Function (1954) by economists Franco Modigliani and Richard Brumberg, proposes that households’ spending decisions are driven by household members’ assessments of expenditure needs and income over the remainder of their lives, taking into account predictable events such as a precipitous drop in income at retirement. The standard version of the life-cycle model also assumes that consumers would prefer to spend everything before they die (i.e., it assumes there is no bequest motive). Life-cycle models are most commonly employed by microeconomists modeling household-level data on consumption, income, or wealth.

Macroeconomists tend to use a simplified version of the optimization framework called the “permanent income hypothesis,” whose origins trace back to economist Milton Friedman’s treatise A Theory of the Consumption Function (1957). The permanent income hypothesis omits the detailed treatment of demographics and retirement encompassed in the life-cycle model, focusing instead on the aspects that matter most for macroeconomic analysis, such as predictions about the nature of the consumption function, which relates consumer spending to factors such as income, wealth, interest rates, and the like.

Perhaps the most important feature of the consumption function for macroeconomics is what it has to say about the marginal propensity to consume (MPC) when there are changes in income. Economist John Maynard Keynes, who was the first to stress the importance of the MPC in The General Theory of Employment, Interest, and Money (1936), believed that up to 90 percent of any increase in current income would translate into an immediate increase in consumption expenditure (an MPC of 90 percent). However, evidence has shown that Friedman’s permanent income hypothesis is much nearer the mark: Friedman asserted that on average only about one-third of any windfall (a one-time unanticipated gain) would be spent within a year. He further argued that a one-for-one correlation between increased income and increased spending would occur only when the income increase was perceived to reflect a permanent change in circumstances (e.g., a new, higher-paying job).

The modern mathematical versions of the life-cycle and permanent-income-hypothesis models used by most economists bring some plausible refinements to the original ideas. For example, the modern models imply that the marginal propensity to consume out of windfalls is much higher for poor than for rich households. This tendency makes it impossible to determine the impact of a tax cut or government program on consumption spending without knowing whether it is aimed primarily at low-wealth or high-wealth households. The theory further indicates that tax cuts or spending programs (such as extended unemployment benefits) aimed primarily at lower-income households should be considerably more effective at stimulating or maintaining aggregate spending than programs aimed at richer households.

In The Wealth of Nations (1776), Scottish economist Adam Smith wrote:

A linen shirt…is, strictly speaking, not a necessary of life. The Greeks and Romans lived, I suppose, very comfortably though they had no linen. But in the present times, through the greater part of Europe, a creditable day-labourer would be ashamed to appear in public without a linen shirt, the want of which would be supposed to denote [a] disgraceful degree of poverty.

Smith clearly did not believe one of the baseline assumptions built into the standard models of consumption described above: that the pleasure yielded by a given level of consumption is independent of the consumption standards of the surrounding community. A day labourer in Smith’s time was a consumer of linen shirts for social as well as practical reasons. However, research into the consequences of this type of “comparison utility” suggests that observable individual spending behaviour is much the same whether one cares about absolute or relative levels of consumption, because there is nothing that the typical individual can do to change the consumption levels of others.

If, however, the pleasure yielded by an individual’s current consumption depends partly on a comparison to that person’s past consumption habits, then rational consumers will realize that they will be happier if they increase their level of consumption gradually over their lifetimes (instead of equalizing consumption at different ages, as the life-cycle model suggests). Habit formation also implies a very different reaction to income shocks that reflects a gradual adaptation to new circumstances. The speed of adjustment depends on the strength of the habit. This is in contrast to Friedman’s permanent income hypothesis, which assumes that a permanent shock (either negative or positive) will result in an immediate and complete adjustment of spending. A considerable amount of evidence from macroeconomic data seems to suggest that consumption does indeed react sluggishly to macroeconomic shocks.

A final modification commonly made to the baseline life-cycle model is the abandonment of the assumption that people accumulate wealth solely to finance their own future spending. The high saving rates of the richest few percent of households (at least in the United States) are hard to explain in such a framework. The most popular solution is to incorporate a “bequest motive,” which explains the high saving rates of the very rich as resulting from beneficence toward their descendants. An alternative theory holds that some rich people gain satisfaction directly from the ownership of wealth, not merely from the happy contemplation of that wealth being spent by children, grandchildren, and so on. People might enjoy being wealthy for reasons of status, power, avarice, or other motivations that fall outside the traditional scope of economic analysis.

The modifications just described pose no challenge to the premise that consumers are fully informed rational optimizers. The popularity of this assumption reflects the fact that there is usually only one way to behave rationally, but there are a great many possible ways to behave stupidly. In the absence of a general theory of stupidity, economists have been unable to construct a unified, compelling alternative to the rational optimization framework.

Nonetheless, a few specific deviations from fully informed rationality have been explored. Evidence from experimental psychology suggests that people have difficulty resisting the impulse for instant gratification, even when they agree (at any time other than the exact moment of temptation) that it would be rational to resist. Whether such self-control problems have large economic effects is unclear. Economists have developed models showing that self-control problems have minor consequences if it is possible for consumers to make commitments that are difficult or troublesome to reverse—such as having an employer deduct a specified portion of an employee’s paycheck for retirement savings before the money is deposited the employee’s bank account (see 401(k)). It turns out that if such commitment strategies are available, they permit the consumer to achieve a lifetime consumption pattern very close to that predicted by the standard rational optimizing model, which makes no allowances for self-control problems or commitment mechanisms.

Some distinctive implications, however, emerge in the model built on self-control problems. In particular, this model can explain seemingly illogical behaviour, such as holding a retirement savings account earning an interest rate of, say, 7 percent while simultaneously borrowing on credit cards and paying interest rates of up to 20 percent. Either the saving or the borrowing can be justified in the rational optimization framework (if we assume that savers are rational and patient while borrowers are rational but impatient), but the simultaneous practice of saving at a low interest rate and borrowing at a high interest rate is virtually impossible to reconcile within the rationality framework. This practice can be explained, however, by assuming that credit-card borrowing is the result of (largely) irrational impulse spending, while the retirement saving deductions are assumed to have been set up by the cool-headed, rational side of the consumer’s nature.

One other well-explored category of deviation from the standard framework simply drops the assumption that people are fully informed. Consumers who do not know whether a given shock to their incomes is transitory or permanent will tend to react as though there is some chance it is temporary and some chance that it is permanent. Alternatively, consumers who do not pay much attention to macroeconomic news may be slow to react when macroeconomic circumstances change (e.g., a recession). This category of theories may provide an alternative to habit formation as a means of explaining the sluggish reaction of consumption spending to economic news.