Intertemporal consumption

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Economic theories of intertemporal consumption seek to explain people's preferences in relation to consumption and saving over the course of their lives. The earliest work on the subject was by Irving Fisher and Roy Harrod, who described 'hump saving', hypothesizing that savings would be highest in the middle years of a person's life as they saved for retirement.

In the 1950s, more well-defined models were built on discounted utility theory and approached the question of inter-temporal consumption as a lifetime income optimization problem. Solving this problem mathematically, assuming that individuals are rational and have access to complete markets, Modigliani & Brumberg (1954), Albert Ando, and Milton Friedman (1957) developed what became known as the life-cycle model. See Intertemporal choice § Modigliani's life cycle income hypothesis for details.

The life-cycle model of consumption suggests that consumption is based on average lifetime income instead of income at any given age. First, young people borrow to consume more than their income, next, as their income rises through the years, their consumption rises slowly and they begin to save more. Lastly, during their retirement these individuals live off of their savings. Furthermore this theory implies that consumption is smoothed out relative to a person's income which is the reason economists set consumption proportional to potential income rather than actual income.

Attempts to test the life-cycle model against real world data have met with mixed success. In a review of the literature, Courant, Gramlich and Laitner (1984) note "but for all its elegance and rationality, the life-cycle model has not tested out very well". The main discrepancies between predicted and actual behaviour is that people drastically 'underconsume' early and late in their lifetime by failing to borrow against future earnings and not saving enough to adequately finance retirement incomes respectively. People also seem to 'overconsume' during their highest earning years, the elderly do not consume from their assets as would be expected (particularly from their household equity) and also treat windfall gains in a manner inconsistent with the life-cycle model. Specific alterations to the theory have been proposed to help it accommodate the data; a bequest motive, capital market imperfections such as liquidity constraints, a changing individual utility function over time or a particular form of expectation as to future income.

Behavioural economists have proposed an alternate description of intertemporal consumption, the behavioural life cycle hypothesis. They propose that people mentally divide their assets into non-fungible mental accounts – current income, current assets (savings) and future income. The marginal propensity to consume (MPC) out of each of these accounts is different. Drawing upon empirical studies of consumption, superannuation and windfall gains they hypothesize that the MPC is close to one out of current income, close to zero for future income and somewhere in between with respect to current assets. These differing MPCs explain why people 'overconsume' during their highest earning years, why increasing superannuation contributions does not cause current savings to be reduced (as the life-cycle model implies) and why small windfall gains (which are coded as current income) are consumed at a high rate but a higher proportion of larger gains is saved.

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Franco Modigliani Italian-American economist (1918–2003)

Franco Modigliani was an Italian-American economist and the recipient of the 1985 Nobel Memorial Prize in Economics. He was a professor at University of Illinois at Urbana–Champaign, Carnegie Mellon University, and MIT Sloan School of Management.

Saving Income which is not immediately spent or otherwise used for consumption

Saving is income not spent, or deferred consumption. Methods of saving include putting money aside in, for example, a deposit account, a pension account, an investment fund, or as cash. Saving also involves reducing expenditures, such as recurring costs. In terms of personal finance, saving generally specifies low-risk preservation of money, as in a deposit account, versus investment, wherein risk is a lot higher; in economics more broadly, it refers to any income not used for immediate consumption. Saving does not automatically include interest.

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Intertemporal choice is the process by which people make decisions about what and how much to do at various points in time, when choices at one time influence the possibilities available at other points in time. These choices are influenced by the relative value people assign to two or more payoffs at different points in time. Most choices require decision-makers to trade off costs and benefits at different points in time. These decisions may be about saving, work effort, education, nutrition, exercise, health care and so forth. Greater preference for immediate smaller rewards has been associated with many negative outcomes ranging from lower salary to drug addiction.

In economics, time preference is the current relative valuation placed on receiving a good or some cash at an earlier date compared with receiving it at a later date.

The marginal propensity to save (MPS) is the fraction of an increase in income that is not spent and instead used for saving. It is the slope of the line plotting saving against income. For example, if a household earns one extra dollar, and the marginal propensity to save is 0.35, then of that dollar, the household will spend 65 cents and save 35 cents. Likewise, it is the fractional decrease in saving that results from a decrease in income.

Consumption (economics) Using money to obtain an item for use

Consumption is the act of using resources to satisfy current needs and wants. It is seen in contrast to investing, which is spending for acquisition of future income. Consumption is a major concept in economics and is also studied in many other social sciences.

In economics, the marginal propensity to consume (MPC) is a metric that quantifies induced consumption, the concept that the increase in personal consumer spending (consumption) occurs with an increase in disposable income. The proportion of disposable income which individuals spend on consumption is known as propensity to consume. MPC is the proportion of additional income that an individual consumes. For example, if a household earns one extra dollar of disposable income, and the marginal propensity to consume is 0.65, then of that dollar, the household will spend 65 cents and save 35 cents. Obviously, the household cannot spend more than the extra dollar. The MPC is higher in the case of poorer people than in rich.

Consumption function

In economics, the consumption function describes a relationship between consumption and disposable income. The concept is believed to have been introduced into macroeconomics by John Maynard Keynes in 1936, who used it to develop the notion of a government spending multiplier.

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Average propensity to consume (as well as the marginal propensity to consume) is a concept developed by John Maynard Keynes to analyze the consumption function, which is a formula where total consumption expenditures (C) of a household consist of autonomous consumption (Ca) and income (Y) multiplied by marginal propensity to consume. According to Keynes, the individual´s real income determines saving and consumption decisions.

In economics, the life-cycle hypothesis (LCH) is a model that strives to explain the consumption patterns of individuals.

Permanent income hypothesis Economic model explaining consumption pattern formation

The permanent income hypothesis (PIH) is a model in the field of economics to explain the formation of consumption patterns. It suggests consumption patterns are formed from future expectations and consumption smoothing. The theory was developed by Milton Friedman and published in his A Theory of Consumption Function, published in 1957 and subsequently formalized by Robert Hall in a rational expectations model. Originally applied to consumption and income, the process of future expectations is thought to influence other phenomena. In its simplest form, the hypothesis states changes in permanent income, rather than changes in temporary income, are what drive changes in consumption.

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Consumption smoothing is the economic concept used to express the desire of people to have a stable path of consumption. People desire to translate their consumption from periods of high income to periods of low income to obtain more stability and predictability. There exist many states of the world, which means there are many possible outcomes that can occur throughout an individual's life. Therefore, to reduce the uncertainty that occurs, people choose to give up some consumption today to prevent against an adverse outcome in the future. In order for one to adequately and properly prepare for unforeseen circumstances that can occur in the future, we must start planning today, putting money aside for when these unforeseen circumstances happen.

Elasticity of intertemporal substitution is a measure of responsiveness of the growth rate of consumption to the real interest rate. If the real interest rate rises, current consumption may decrease due to increased return on savings; but current consumption may also increase as the household decides to consume more immediately, as it is feeling richer. The net effect on current consumption is the elasticity of intertemporal substitution.

The random walk model of consumption was introduced by economist Robert Hall. This model uses the Euler numerical method to model consumption. He created his consumption theory in response to the Lucas critique. Using Euler equations to model the random walk of consumption has become the dominant approach to modeling consumption.

Precautionary saving is saving that occurs in response to uncertainty regarding future income. The precautionary motive to delay consumption and save in the current period rises due to the lack of completeness of insurance markets. Accordingly, individuals will not be able to insure against some bad state of the economy in the future. They anticipate that if this bad state is realized, they will earn lower income. To avoid adverse effects of future income fluctuations and retain a smooth path of consumption, they set aside a precautionary reserve, called precautionary savings, by consuming less in the current period, and resort to it in case the bad state is realized in the future.

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