ECON1002 · Introductory Macroeconomics
Saving, Wealth & Investment
This chapter separates a flow (saving = current income − current spending) from a stock (wealth = assets − liabilities), then defines national saving S = Y − C − G and splits it into private and public saving. The firm's investment decision follows cost-benefit logic (invest while marginal benefit ≥ marginal cost, where the real interest rate is the cost of funds), and the saving-investment market brings the two together with the real interest rate clearing them.
It is examined as MCQ classification (flow vs stock) and as a saving-investment diagram showing how a budget deficit raises the real rate and crowds out private investment.
What this chapter covers
- 011. Saving = income − spending (a FLOW); wealth = assets − liabilities (a STOCK)
- 022. Classifying flows vs stocks (investment, income, deficit, hires = flows; capital, wealth, debt = stocks)
- 033. Why people save: life-cycle, precautionary, bequest motives
- 044. National saving S = Y − C − G; split S = (Y − T − C) + (T − G) = private + public saving
- 055. Government budget: T > G surplus (public saving > 0); T < G deficit (public saving < 0)
- 066. Closed-economy identity S = I
- 077. The investment decision: invest while MB (the MPK / value of marginal product of capital) ≥ MC (real interest rate)
- 088. The saving-investment market: real interest rate r clears S and I; a deficit crowds out private investment
National saving, its split, and the closed-economy identity
- 1 mark(a) National saving is output not consumed by households or government: S = Y − C − G = 1000 − 620 − 180 = 200.
- 1 mark(b) Private saving is disposable income minus consumption: Sₚᵣᵢᵥ = Y − T − C = 1000 − 200 − 620 = 180.
- 1 mark(c) Public saving is the budget balance: S_gov = T − G = 200 − 180 = 20 (a surplus). Check: 180 + 20 = 200 = S. ✓
- 1 mark(d) In a closed economy saving equals investment: I = S = 200.
Key terms
- Flow vs stock
- A flow is measured per unit of time (saving, income, the budget deficit, new hires, investment); a stock is measured at a point in time (wealth, the capital stock, public debt, a bank balance). Saving is the flow that, accumulated over time, changes the stock of wealth.
- National saving (S = Y − C − G)
- The part of total output not consumed privately or by government. It equals the resources available to fund investment and, in a closed economy, S = I.
- Private vs public saving
- Private saving = Y − T − C (disposable income not consumed); public saving = T − G (the government budget balance, positive in surplus, negative in deficit). Their sum is national saving.
- Real interest rate as the cost of capital
- The real interest rate is the opportunity cost of funds tied up in an investment project, so a firm invests while the value of capital's marginal product (MB) is at least the real rate (MC). A higher real rate reduces desired investment.
- Crowding out
- When government borrowing (a budget deficit) raises the real interest rate, private investment falls because fewer projects clear the higher cost of funds. The deficit 'crowds out' private capital formation.
- Saving-investment market
- The market in which national saving (supply of funds, upward in r) meets desired investment (demand for funds, downward in r), with the real interest rate adjusting to clear it. Shifts in either curve change the equilibrium real rate and quantity of investment.
Saving, Wealth & Investment FAQ
How is this material examined in ECON1002?
Two ways: a conceptual MCQ that asks you to classify items as flows or stocks (saving, deficit and income are flows; wealth, capital and debt are stocks), and a saving-investment diagram or calculation showing how a budget deficit raises the real interest rate and crowds out private investment. Computing national saving and its private/public split from Y, C, G and T is the core arithmetic.
What is the difference between saving and wealth?
Saving is a flow — the part of this period's income you do not spend — measured over a period. Wealth is a stock — your assets minus your liabilities — measured at a moment. Positive saving adds to wealth over time, so saving is the flow that fills (or drains) the stock of wealth. The classic MCQ trap is calling wealth a flow or saving a stock.
How does a budget deficit crowd out investment?
A deficit means public saving (T − G) is negative, lowering national saving. With less saving supplied to the saving-investment market, the real interest rate rises to ration funds, and at the higher rate some private investment projects no longer clear MB ≥ MC. So government borrowing partly displaces private capital formation — crowding out.
When does a firm decide to invest?
A firm invests while the marginal benefit of an extra unit of capital — the value of its marginal product — is at least the marginal cost, which is the real interest rate (the opportunity cost of the funds) plus the price of the equipment. As the real rate rises, fewer projects clear this hurdle, so desired investment falls, giving the downward-sloping investment demand in the saving-investment market.
Exam move
Lock in the two identities first: national saving S = Y − C − G, and its split into private (Y − T − C) and public (T − G) saving — practise until the T-terms cancelling is obvious, and always check that your two parts add back to S. Memorise a short list of flows vs stocks so the classification MCQ is instant. For the investment side, internalise that a firm invests while MB (value of capital's marginal product) ≥ MC (the real interest rate), which makes investment demand slope down in r. Then rehearse the crowding-out story on the saving-investment diagram: a deficit lowers saving, raises the real rate, and reduces private investment — and be able to state that one-line mechanism alongside the graph.
