Consumption and Savings Theory in Macroeconomics

Consumption and Savings Theory is a core component of macroeconomics. Its core research focus is the decision-making mechanisms and influencing factors of household consumption and savings, as well as the interrelationship between these two variables and national income and economic growth. Consumption is an important part of aggregate demand, while savings are the source of capital accumulation. The trade-off relationship between consumption and savings directly affects short-term economic fluctuations and long-term economic growth. The core analytical starting point of this theory is the basic relationship between income, consumption and savings: $Y = C + S$ (where $Y$ is disposable income, $C$ is consumption, and $S$ is savings).

I. Keynes' Absolute Income Hypothesis

Keynes' Absolute Income Hypothesis is the earliest modern consumption theory, put forward in The General Theory of Employment, Interest and Money. Its core emphasizes that consumption is determined by current absolute disposable income.

1. Core Views

1. Law of Diminishing Marginal Propensity to Consume (MPC)
   The marginal propensity to consume refers to the increase in consumption caused by each additional unit of disposable income, i.e., $MPC=\frac{\Delta C}{\Delta Y}$.
   Keynes argued that the marginal propensity to consume is greater than 0 and less than 1, and shows a diminishing trend as income increases — low-income groups allocate a higher proportion of their income increments to consumption, while high-income groups tend to save more.

2. Law of Diminishing Average Propensity to Consume (APC)
   The average propensity to consume refers to the proportion of consumption in disposable income, i.e., $APC=\frac{C}{Y}$.
   The average propensity to consume also decreases with the increase of income, and is greater than the marginal propensity to consume in numerical value ($APC>MPC$).

3. Form of the Consumption Function
   Keynes' consumption function can be expressed in a linear form:
   $$C = C_0 + cY_d$$
   Among them, $C_0$ is autonomous consumption (consumption that must be carried out even if income is 0, such as food, clothing, housing and transportation for subsistence), $c$ is the marginal propensity to consume, and $Y_d$ is disposable income.

2. Policy Implications

Keynes believed that insufficient consumption is an important cause of insufficient effective demand and economic recession. Therefore, the government should increase the income of low-income groups through policies such as transfer payments and tax cuts, because this group has a higher marginal propensity to consume and can stimulate aggregate demand more effectively.

3. Limitations

This hypothesis is based on short-term static analysis and cannot explain consumption phenomena in long-term economic development. For example, in the long run, household income has increased significantly, but the average propensity to consume has not shown a continuous downward trend, which is inconsistent with the conclusion of the hypothesis.

II. Duesenberry's Relative Income Hypothesis

Duesenberry's Relative Income Hypothesis breaks through the limitation that "consumption is only determined by current income", and introduces the "demonstration effect" and "ratchet effect" of consumption.

1. Core Views

1. Demonstration Effect
   Household consumption decisions are not isolated, but are affected by the consumption level of surrounding groups. Low-income groups tend to imitate the consumption behavior of high-income groups. Even if their own income is low, they will maintain a relatively high consumption level by reducing savings. This means that consumption depends not only on their own income, but also on the relative income level.

2. Ratchet Effect
   Consumption has irreversibility, that is, consumption tends to rise with the increase of income, but it is difficult to decline with the decrease of income. When an economic recession leads to a decline in household income, residents will give priority to maintaining the consumption level they have already achieved, by reducing savings or even borrowing, so that the short-term propensity to consume is higher than the long-term propensity to consume.

2. Policy Implications

This hypothesis explains why consumption does not decline sharply during an economic recession. It also shows that increasing the overall household income level and narrowing the income gap can improve the overall social propensity to consume through the demonstration effect.

III. Friedman's Permanent Income Hypothesis

Friedman's Permanent Income Hypothesis divides household income into permanent income and transitory income, with the core view that consumption is determined by permanent income.

1. Core Concepts

1. Permanent Income
   Refers to the stable income that residents can obtain in the long run, which is the long-term trend value of income, such as fixed wages and rental income. Residents can estimate the level of permanent income by observing past income and expecting the future.

2. Transitory Income
   Refers to occasional and unstable income, such as bonuses, lottery winnings, and temporary part-time income, which is characterized by randomness and temporariness.

2. Core Views

1. Consumption has a stable functional relationship with permanent income: $C = cY_p$, where $Y_p$ is permanent income, and $c$ is the marginal propensity to consume of permanent income (with a stable value).
2. The marginal propensity to consume of transitory income is close to 0 — residents will not significantly increase consumption because they accidentally obtain a sum of income, but tend to convert transitory income into savings.
3. Residents will only increase their consumption level when they believe that the increase in transitory income will be converted into permanent income. For example, a one-time bonus will only lead to an increase in consumption if it is regarded as a long-term wage increase.

2. Policy Implications

Temporary tax cuts or transfer payments have limited effect on stimulating consumption. Only long-term and stable income growth policies can effectively increase household consumption.

IV. Modigliani's Life-Cycle Hypothesis

Modigliani's Life-Cycle Hypothesis analyzes consumption behavior from the perspective of households' lifetime income and consumption planning, with the core idea that residents will smooth consumption at all stages according to their expected lifetime income.

1. Core Assumptions

A resident's life can be divided into three stages: youth (low income, consumption greater than income, borrowing for consumption), middle age (high income, income greater than consumption, increasing savings), and old age (low income, consumption greater than income, using savings).

2. Core Views

1. Form of the consumption function: $C = \alpha WR + \beta Y_d$
   Among them, $WR$ is wealth stock (such as savings and real estate), $\alpha$ is the marginal propensity to consume of wealth; $Y_d$ is current disposable income, and $\beta$ is the marginal propensity to consume of income.
2. Household consumption decisions depend on total lifetime wealth (including the present value of current income and expected future income), rather than current income.
3. From a social perspective, population structure will affect the total social propensity to consume — when the proportion of the elderly population is too high, the social savings rate will decline; when the proportion of working-age population is high, the social savings rate will rise.

3. Policy Implications

1. Improving the social security system (such as the pension system) can reduce residents' precautionary savings and increase current consumption.
2. Population aging will reduce the social savings rate, which in turn affects capital accumulation and economic growth. It is necessary to optimize the population structure through policies such as delaying retirement and encouraging childbirth.

V. Influencing Factors of Consumption and Savings

In addition to income factors, household consumption and savings decisions are also affected by the following factors:

1. Interest Rate Level
   The impact of interest rates on savings has substitution effect and income effect: the substitution effect means that when interest rates rise, the return on savings increases, and residents tend to reduce consumption and increase savings; the income effect means that when interest rates rise, residents' interest income increases, and they may increase consumption and reduce savings. The final impact depends on the strength of the two effects.
2. Price Level and Inflation Expectations
   Rising prices will reduce residents' real income and restrain consumption; if residents expect inflation to intensify in the future, they will tend to increase current consumption and reduce savings.
3. Precautionary Savings Motivation
   Uncertainty about future income (such as the risk of unemployment) and uncertainty about expenditure (such as medical and educational expenditure) will prompt residents to increase precautionary savings and reduce current consumption.
4. Wealth Level
   Rising prices of assets such as real estate and stocks will increase residents' wealth stock, improve the marginal propensity to consume of wealth, and then stimulate consumption (i.e., the wealth effect).

VI. The Relationship Between Consumption, Savings and Economic Growth

1. Short-term Perspective: Consumption is a core part of aggregate demand. An increase in consumption will directly drive the growth of total output; an increase in savings will reduce current consumption and may lead to insufficient effective demand.
2. Long-term Perspective: Savings are the source of capital accumulation. After savings are converted into investment, they will increase the capital stock, improve production efficiency, and then promote long-term economic growth. Therefore, long-term economic growth requires a moderate savings rate — an excessively low savings rate will lead to insufficient capital accumulation, while an excessively high savings rate will restrain current consumption and affect short-term demand.