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In economics, the consumption function describes a relationship between consumption and disposable income. [ 1 ] [ 2 ] 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 .
Average propensity to consume (APC) (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 (C a) and income (Y) (or disposable income (Y d)) multiplied by marginal propensity to consume (c 1 or MPC).
This has led to the absolute income hypothesis falling out of favor as the consumption model of choice for economists. [3] Keynes' consumption function has come to be known as 'absolute income hypothesis' or 'absolute income theory'. His statement of the relationship between income and consumption was based on psychological law.
Keynesian economics developed during and after the Great Depression from the ideas presented by Keynes in his 1936 book, The General Theory of Employment, Interest and Money. [5] Keynes' approach was a stark contrast to the aggregate supply-focused classical economics that preceded his book.
In the simplest exposition of Keynesian theory, the economy is assumed to be closed (which implies that NX = 0), and planned investment is exogenous and determined by the animal spirits of investors. Consumption is an affine function of income, C = a + bY where the slope coefficient b is called the marginal propensity to consume.
In a standard Keynesian model, the MPC is less than the average propensity to consume (APC) because in the short-run some (autonomous) consumption does not change with income. Falls (increases) in income do not lead to reductions (increases) in consumption because people reduce (add to) savings to stabilize consumption.
In Keynesian macroeconomics, the Fundamental Psychological Law underlying the consumption function states that marginal propensity to consume (MPC) and marginal propensity to save (MPS) are greater than zero(0) but less than one(1) MPC+MPS = 1 e.g. Whenever national income rises by $1 part of this will be consumed and part of this will be saved
The Keynesian consumption function is also known as the absolute income hypothesis, as it only bases consumption on current income and ignores potential future income (or lack of). Criticism of this assumption led to the development of Milton Friedman 's permanent income hypothesis and Franco Modigliani 's life cycle hypothesis .