ECON2000 Chap.7 Consumption and Investment
Consumption and Investment
Consumption responds to current disposable income, expected lifetime resources, wealth and access to credit. Life-cycle and permanent-income theories explain smoothing, while hand-to-mouth households show why the distribution of an income change affects aggregate demand.
Investment responds negatively to the real interest rate, positively to expected profitability and to Tobin's q, and may be limited by financing constraints even when a project appears worthwhile.
This chapter compares a current-income consumption rule with longer-horizon theories, uses heterogeneous marginal propensities to calculate the immediate effect of transfers and translates household and firm decisions into shifts of planned expenditure and the IS curve.
What this chapter covers
- 01
Consumption function
- 02
Marginal propensity to consume
- 03
Life-cycle hypothesis
- 04
Permanent income
- 05
Hand-to-mouth households
- 06
Real rates and investment
- 07
Tobin's q
- 08
Financing constraints
Worked example · free
A transfer across heterogeneous households
- +1The constrained household raises consumption by 900.
- +1The smoothing household raises consumption by 250.
- +1Total immediate consumption rises by 1,150.
- +2The same total transfer would produce a different aggregate response if allocated between households with different MPCs.
Key terms
- Permanent income
- Expected sustainable resources over time that support a smoothed consumption path.
- Life-cycle hypothesis
- The view that households plan consumption and saving across stages of their lives.
- Hand-to-mouth household
- A household whose consumption closely follows current cash income because smoothing is constrained.
- Tobin's q
- The market value of installed capital relative to the cost of replacing it.
- Financing constraint
- A limit preventing a household or firm from borrowing on the terms assumed by unconstrained models.
Consumption and Investment FAQ
Why does a permanent income change affect consumption more?
A permanent change alters expected lifetime resources by more than a temporary payment of the same initial size. A household that smooths consumption spreads that larger resource change across time, producing a bigger sustained consumption response.
Why do hand-to-mouth households matter for policy?
They have high marginal propensities to consume from current cash income because borrowing or liquid saving is limited. Transfers directed toward them can therefore produce a larger immediate aggregate-demand effect than equal transfers to households that smooth spending.
Why does investment fall when real rates rise?
A higher real rate raises financing costs and the return a project must beat. Fewer projects have positive present value, so planned investment falls. The relevant rate is real because expected inflation changes the purchasing-power cost of borrowing.
What does Tobin's q reveal?
A q above one means markets value installed capital above replacement cost, strengthening the incentive to build more. A q below one weakens that incentive. Adjustment costs, expectations and financing constraints can prevent a mechanical one-for-one response.
Exam move
Place each consumption theory beside the income concept it uses: current disposable income, expected lifetime resources or liquid cash-on-hand. Work transfer examples with at least two household MPCs so aggregate effects are not treated as distribution-free. For investment, write the real interest rate, expected payoff, q and financing constraint as separate channels.
When translating to IS, identify whether the shock shifts autonomous spending or changes interest sensitivity. Check nominal rates for expected inflation before using them in an intertemporal decision. End each policy answer by naming which households or firms are constrained, because the representative-agent shortcut may hide the main transmission mechanism.
Build paired scenarios for a temporary bonus and a permanent salary rise, holding the first payment equal, then explain why a smoothing household reacts differently. Add a constrained household with no liquid buffer and calculate the aggregate response under alternative transfer allocations. For investment, rank several projects by expected return and remove those falling below the real financing cost.
Change q and collateral separately so market valuation is not confused with access to finance. Translate each shock into the planned-expenditure block: wealth and confidence change autonomous spending, while the real rate changes interest-sensitive spending. Include an uncertainty case where households save defensively and firms delay irreversible projects even before current income falls.
A high-quality answer names the horizon, the constraint and the expectation that drives the decision. This makes the movement in consumption or investment traceable rather than merely asserting that confidence changed. Create a matrix with temporary versus permanent income changes across constrained versus unconstrained households. Predict consumption before calculating it.
For firms, compare a rate increase, a fall in expected sales, a decline in q and a tightening collateral constraint; each reduces investment through a different route. Translate each route into an AE or IS statement. The matrix prevents every spending change from being labelled loosely as confidence and preserves the underlying economic mechanism.
Working through Consumption and Investment in ECON2000? Sia is AskSia’s AI Economics tutor — ask any ECON2000 Consumption and Investment question and get a clear, step-by-step explanation grounded in how ECON2000 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.