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Intertemporal Approach to the Current Account. GDP vs. GNP. GDP is value of production/ value of income produced within a domestic economy. GNP is value of income earned by residents of domestic economy. GNP = GDP + NFP

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gdp vs gnp
  • GDP is value of production/ value of income produced within a domestic economy.
  • GNP is value of income earned by residents of domestic economy.


  • Net factor payments is income earned from overseas sources by domestic residents less income earned from domestic sources by overseas residents.
current account
Current Account
  • The current account is, conceptually, the amount of income earned overseas less the amount of income earned by foreigners from the domestic economies: CA = NX + NFP.
  • Private Savings is disposable income minus consumption spending.

SP = (Y + NFP – T – C)

  • Public savings is the budget surplus (T- G)
  • Capital investment and the budget deficit can be financed by domestic savings or net borrowing from abroad.

SP + KA = I + (G-T)

capital financial account
Capital & Financial Account
  • The capital account (more accurately the capital & financial account) records capital inflows into the country. The account includes the financial account, the capital account, and change in reserve assets.
current account negative of capital account
Current Account = Negative of Capital Account
  • S = Y + NFP – T – C +KA = I + (G-T)

-KA = Y+NFP – I – C – G = [Y - I- C - G] +NFP

  • Y = C + I + G + NX→ NX = [Y - I- C - G]

-KA = NX + NFP

  • Intuition: When you buy more from overseas than you earn overseas from exports or other income, you must borrow to make up the difference
hong kong current account capital account 2001
Hong Kong Current Account & Capital Account 2001
  • Hong Kong had a 96 million dollar current account surplus in 2001.
  • Hong Kong had a 97 million dollar capital & financial account deficit.
  • The difference is reserve assets.
current account equals the accumulation of wealth
Current account equals the accumulation of wealth
  • Define Net International Investment Position, NIIPt, as a country’s foreign wealth less foreign owned domestic assets including stocks, bonds, real estate etc.
  • NIIPt is end of period wealth accumulateds through current account.

NIIPt – NIIPt-1 = CAt = NXt + NFPt

  • Assume that NFP is real investment income (which for the most part is a reasonable assumption). NFPt = r∙NIIPt-1

NIIPt = (1+r)NIIPt-1 +CAt

wealth to gdp ratio
Wealth to GDP ratio
  • Divide both sides of the wealth accumulation equation by Yt
  • If current account (as a share of GDP) is below some level it will lead to a loss of wealth relative to GDP and if the ultimately increasing debt.
sustainable current account
Sustainable current account
  • A large, perpetually negative current account is unsustainable as it would result in infinite debt.
  • Sustainable current account is a current account that would maintain net international investment position at a constant level
  • Define the sustainable current account
determinants of the current account
Determinants of the Current Account
  • Current account is the gap between domestic savings and the domestic investment and the budget deficit.

CA = S – I – (G-T)

  • Examine the determinants of each in turn.
consider a firm considering in owning 1 worth of capital for 1 period
Consider a firm considering in owning $1 worth of capital for 1 period.
  • Cost: 1+r (Either borrow funds for 1 period or use own savings and lose chance to get savings.
  • Benefit: Can produce MPK extra goods plus sell any of the capital that hasn’t depreciated (1-d)
  • Firm adds to profits if benefit is greater than cost. Invest if:
investment choice
Investment Choice

Example: Cobb-Douglas

  • Increases profits to invest as long as MPK > r+d, but investing pushes down MPK. Investment will occur until MPK = r+d.
  • Firms must invest until they reach target capital stock.
volatile investment
Volatile Investment
  • Investment is the most volatile part of demand. A 1% change in technology or increase in employment will increase target capital stock.
  • In any year, investment is one-tenth as large as the capital stock.
    • A 1% change in the capital stock requires a 10% change in investment.
investment curve
Investment Curve


Q: Why does the curve slope down? The greater is interest rate, the more profitable capital must be to invest in it.

Q: What shifts the curve? Increases in technology and labor increases profitability of capital.


two consumption theories
Two Consumption Theories
  • Keynesian: Consumption is dependent on current income.
  • Permanent Income Theory: Consumption decision is a savings decision so households take into account future income as well as outstanding financial wealth.
    • People prefer smooth consumption and save and borrow to do so.
why do people save
Why do People Save?
  • Life Cycle Motives – Income is Not Smooth Across Time. Households save, in part, to transfer income from high income periods to low income periods.
  • Precautionary Motives – Households like to achieve a buffer stock of wealth in the case of a possible bad outcome. If households have a buffer stock of saving, bad outcomes in terms of income don’t result in really bad outcomes in terms of consumption.
life cycle motives two period model
Life Cycle Motives: Two Period Model
  • To examine life-cycle theory, we use simplest possible model.
  • One good consumed by a household that lives two periods, C0 and C1.
  • Household lives and earns income Y0 and Y21 in each period.
  • Household pays taxes in each period T0 and T1.
  • Household can buy/sell bonds, B, at real interest rate r.
temporal budget constrants
Temporal Budget Constrants
  • First period,

B0 = Y0 – T0 – C0 1)

  • Second period,

C1=Y1 –T1+(1+r)B0 2)

  • Note B can be either > or < 0. If B > 0, household is a saver. If B < 0, household is a borrower.
intertemporal budget constraint
Combine two budget constraints

Present Discounted Value of Lifetime Income equals Present Discounted Value of Lifetime Consumption.

Intertemporal Budget Constraint
consider 3 scenarios
Consider 3 scenarios
  • Baseline Y1 = Y2 = Y implies
  • Permanent Consumption Hypothesis


temporary rise in income
Temporary Rise in Income
  • The propensity to consume is a fraction of the temporary extra income. The remainder is saved for the future.
permanent rise in income
Permanent Rise in Income
  • The propensity to consume is larger when the increase in consumption is permanent. There is no need to save a permanent rise in income for the future.
future rise in income
Future Rise in Income
  • Consumption may rise when future income increases which will also increase W. Savings will fall as people borrow to enjoy future income today.
permanent income hypothesis
Permanent Income Hypothesis
  • A simplified (and extreme) version of this theory hypothesizes that consumption is equal in each period.
income stream consumption
Income Stream & Consumption
  • Consider three hypothetical increases in income of Δ.
    • A Temporary Increase – Y1increase by Δ, but Y2 is unchanged. This will increase W by Δ.
    • A Future Increase – Y2increases by 100, but Y1 is unchanged. W increases by Δ /1+r≈ Δ
    • A Permanent Increase – Y1 & Y2increase by 100. W increases by Δ(2+r/1+r) ≈2∙ Δ
  • Cases 1 & 2 increase W by nearly identical amounts. But current consumption depends only on W. Thus, cases 1 & 2 will increase by similar amounts.
  • Case 3 increases W by nearly double the amount.
income stream and savings
Income Stream and Savings
  • In the first case, future income does not rise but optimal future consumption, C2* does . Current savings must rise.
  • In the second case, current income does not rise, but optimal current consumption. Current savings must fall.
  • What happens to savings with a permanent change in income?
application life cycle of saving
Application: Life Cycle of Saving
  • Permanent Income Hypothesis suggests that households like to keep a constant profile of consumption over time.
  • Age profile of income however is not constant. Income is low in childhood, rises during maturity and reaches a peak in mid-1950’s and drops during retirement.
  • This generates a time profile for savings defined as the difference between income and consumption.
time path of savings
Time Path of Savings








east asian demographics
East Asian Demographics
  • During last 25 years, East Asian Nations had a sharp decrease in their ‘dependency ratio’.
  • Dependency ratio is the % of people in their non-working years (children & seniors.
  • Dependents are dis-savers and non-dependents are savers.
east asian demographics38
East Asian Demographics
  • Due to plummeting birth rates, East Asia had a plummeting ratio of youths as a share of population
  • This put a large share of population in high savings years.
  • Share of prime age adults has hit its peak in most Asian countries and will fall over the next half century.
interest rates incentives and effects
Interest Rates: Incentives and Effects
  • A rise in interest rates increases the payoff to savings and increases the incentive to save. Substitution Effect (Plus Factor for All)
  • A rise in the interest rate reduces the amount of savings you need to do to meet target level of future consumption. Income Effect (Minus Factor for Net Savers).
  • A rise in the interest rate reduces the amount of borrowing you can do and still meet some target lever of future consumption. Income Effect (Plus Factor for Net Borrowers)
aggregate savings interest rates
Aggregate Savings & Interest Rates
  • Interest rates have a positive impact on savings by borrowers, i.e. borrowers reduce their borrowing.
  • Interest rates have an ambiguous effect on savings by savers.
  • Since there is positive net savings, interest rates have ambiguous effect on aggregate savings.
  • Empirically, impact of interest rates on savings are hard to detect.
saving curve
Saving Curve


Q: Why does the curve slope up? Empirical work suggests substitution effect is slightly more powerful than income effect.

Q: What shifts the curve? Changes in current income relative to future income.