CONSUMPTION THEORIES

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JOHN MAYNARD KEYNES AND THE CONSUMPTION FUNCTION

The consumption function was central to Keynes’ theory of economic fluctuations presented in The General Theory in 1936.

Keynesian three conjectures:

1.  Keynes conjectured that the marginal propensity to consume– the amount consumed out of an additional dollar of income– is between zero and one. He claimed that the fundamental law is that out of every dollar of earned income, people will consume part of it and save the rest.

2.  Keynes also proposed the average propensity to consume– the ratio of consumption to income– falls as income rises.

3.  Keynes also held that income is the primary determinant of consumption and that the interest rate does not have an important role.

THE CONSUMPTION FUNCTION


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Income

Depends MarginalConsumption on

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Propensity to Households

Spending by Consume (MPC)

Autonomous Consumption

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This consumption function exhibits three properties that Keynes conjectured.

1.  The marginal propensity to consume c is between zero and one.

2.  The average propensity to consume falls as income rises.

3.  Consumption is determined by current income.

AVERAGE PROPENSITY TO CONSUME

APC = C/Y = C/Y + c, As Y rises, C/Y falls, and so the average propensity to consume C/Y falls. Notice that the interest rate is not included in this function.

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MARGINAL PROPENSITY TO CONSUME

To understand the marginal propensity to consume (MPC), consider a shopping scenario. A person who loves to shop probably has a large MPC, let’s say (.99). This means that for every extra rupee he or she earns after tax deductions, he or she spends 99 paisas of it. The MPC measures the sensitivity of the change in one variable (C) with respect to a change in the other variable (Y).

SECULAR STAGNATION AND SIMON KUZNETS

  • During World War II, on the basis of Keynes’ consumption function, economists predicted that the economy would experience what they called secular stagnation, a long depression of infinite duration– unless fiscal policy was used to stimulate aggregate demand.
  • It turned out that the end of the war did not throw the U.S. into another depression, but it did suggest that Keynes’ conjecture that the average propensity to consume would fall as income rose appeared not to hold.
  • Simon Kuznets constructed new aggregate data on consumption and investment dating back to 1869 and whose work would later earn a Nobel Prize.
  • He discovered that the ratio of consumption to income was stable over time, despite large increases in income; again, Keynes’ conjecture was called into question. This brings us to the puzzle.

CONSUMPTION PUZZLE

The failure of the secular-stagnation hypothesis and the findings of Kuznets both indicated that the average propensity to consume is fairly constant over time. This presented a puzzle: why did Keynes’ conjectures hold up well in the studies of household data and in the studies of short time-series, but fail when long time series were examined? Studies of household data and short time-series found a relationship between consumption and income similar to the one Keynes conjectured– this is called the short-run consumption function. But, studies using long time-series found that the APC did not vary systematically with income-this relationship is called the long-run consumption function.

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IRVING FISHER AND INTERTEMPORAL CHOICE

The economist Irving Fisher developed the model with which economists analyze how rational, forward-looking consumers make intertemporal choices– that is, choices involving different periods of time. The model illuminates

  • The constraints consumers face,
  • The preferences they have, and
  • How these constraints and preferences together determine their choices about

consumption and saving. When consumers are deciding how much to consume today versus how much to consume in the future, they face an intertemporal budget constraint, which measures the total resources available for consumption today and in the future.

CONSUMER’S BUDGET CONSTRAINT

Consider the decision facing a consumer who lives for two periods (representing youth & age). He earns Income Y1, Y2 and consumes C1, C2 in both periods respectively (adjusted for inflation). The savings in the first period will be

S = Y1 – C1 In the second period

C2 = (1 + r) S + Y2 Where r is the real interest rate. Remember S can represent either saving or borrowing and the equations hold in both cases.

• If C1 < Y1 consumer is saving S > 0

• If C1 > Y1 consumer is borrowing S < 0 Assume: r (borrowing) = r (saving) Combining the two equations:

C2 = (1 + r) (Y1 – C1) + Y2 Rearranging (1 + r)C1 + C2 = (1 + r) Y1 + Y2 Dividing both sides by 1 + r

So we can say that

  • The consumer’s budget constraint implies that if the interest rate is zero, the budget constraint shows that total consumption in the two periods equals total income in the two periods. In the usual case in which the interest rate is greater than zero, future consumption and future income are discounted by a factor of 1 + r.
  • This discounting arises from the interest earned on savings. Because the consumer earns interest on current income that is saved, future income is worth less than current income.
  • Also, because future consumption is paid for out of savings that have earned interest, future consumption costs less than current consumption.
  • The factor 1/ (1+r) is the price of second-period consumption measured in terms of first-period consumption; it is the amount of first-period consumption that the consumer must forgo to obtain 1 unit of second-period consumption.

Here are the combinations of first-period and second-period consumption the consumer can choose.

Second-Period Consumption Consumer’s budget constraint

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If he chooses a point between A and B, he consumes less than his income in the first period and saves the rest for the second period. If he chooses between A and C, he consumes more that his income in the first period and borrows to make up the difference.

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