What is the National Savings and Investment Identity?

The national saving and investment identity provides a framework for showing the relationships between the sources of demand and supply in financial capital markets. The identity begins with a statement that must always hold true: the quantity of financial capital supplied in the market must equal the quantity of financial capital demanded.

The U.S. economy has two main sources for financial capital: private savings from inside the U.S. economy and public savings.

Total savings = Private savings (S) + Public savings (T – G)

These include the inflow of foreign financial capital from abroad. The inflow of savings from abroad is, by definition, equal to the trade deficit. 

We can write this inflow of foreign investment capital as imports (M) minus exports (X). There are also two main sources of demand for financial capital: private sector investment (I) and government borrowing. Government borrowing in any given year is equal to the budget deficit, which we can write as the difference between government spending (G) and net taxes (T). Let’s call this equation 1.

Quantity supplied of financial capital = Quantity demanded of financial capital

Private savings + Inflow of foreign savings  = Private investment + Government budget deficit

S + (M – X) = I + (G –T)

Governments often spend more than they receive in taxes and, therefore, public savings (T – G) is negative. This causes a need to borrow money in the amount of (G – T) instead of adding to the nation’s savings. If this is the case, we can view governments as demanders of financial capital instead of suppliers. In algebraic terms, we can rewrite the national savings and investment identity like this:

Private investment = Private savings +    Public savings    + Trade deficit I = S + (T – G) + (M – X)

Let’s call this equation 2. We must accompany a change in any part of the national saving and investment identity by offsetting changes in at least one other part of the equation because we assume that the equality of quantity supplied and quantity demanded always holds. If the government budget deficit changes, then either private saving or investment or the trade balance—or some combination of the three—must change as well. 

National Savings and Investment Identity in Budget and Trade Surpluses

The national saving and investment identity must always hold true because, by definition, the quantity supplied and quantity demanded in the financial capital market must always be equal. However, the formula will look somewhat different if the government budget is in deficit rather than surplus or if the balance of trade is in surplus rather than deficit. For example, in 1999 and 2000, the U.S. government had budget surpluses, although the economy was still experiencing trade deficits. When the government was running budget surpluses, it was acting as a saver rather than a borrower, and supplying rather than demanding financial capital. As a result, we would write the national saving and investment identity during this time as:

Quantity supplied of financial capital = Quantity demanded of financial capital Private savings + Trade deficit + Government surplus = Private investment

S + (M – X) + (T – G) = I

Let's call this equation 3. Notice that this expression is mathematically the same as equation 2 except the savings and investment sides of the identity have simply flipped sides.

During the 1960s, the U.S. government was often running a budget deficit, but the economy was typically running trade surpluses. Since a trade surplus means that an economy is experiencing a net outflow of financial capital, we would write the national saving and investment identity as:

Quantity supplied of financial capital  =  Quantity demanded of financial capital

Private savings = Private investment + Outflow of foreign savings + Government budget deficit S = I + (X – M) + (G – T)

Instead of the balance of trade representing part of the supply of financial capital, which occurs with a trade deficit, a trade surplus represents an outflow of financial capital leaving the domestic economy and invested elsewhere in the world.

Quantity supplied of financial capital  =  Quantity demanded of financial capital demand

Private savings = Private investment + Government budget deficit + Trade surplus S = I + (G – T) + (X – M)

We assume that the point to these equations is that the national saving and investment identity always hold. When you write these relationships, it is important to engage your brain and think about what is on the supply and demand side of the financial capital market before you start your calculations.

A rising budget deficit may result in a fall in domestic investment, a rise in private savings, or a rise in the trade deficit. The following modules discuss each of these possible effects in more detail.

Related Topics

• What is Government Spending?
• Autonomous Spending
• Autonomous Consumption
• Fiscal Policy
• Expansionary Fiscal Policy
• Contractionary Fiscal Policy
• Progressive vs Regressive Tax
• Marginal Tax Rates
• Proportional Tax
• Trickle Down Theory
• Discretionary Fiscal Policy
• Automatic Stabilizers
• Effects of Discretionary Policy (Interest Rates & Lags)
• Crowding Out Effect 
• National Debt
• Government Borrowing
• Golden Rule
• Ricardian Equivalence
• Balanced Budget - Deficit and Surplus
• National Debt
• Standardized Employment Budget
• Deficit Hawk
• Austerity
• Twin Deficits
• Fiscal Policy and the Aggregate Supply and Demand Curve
• Stabilization Policy
• Robin Hood Effect
• Ricardo Barro Effect
• Automatic Stabilizers
• Standardized Employment Budget
• How Does Fiscal Policy Affect Interest Rates? 
• Crowding Out
• Types of Lag in Fiscal Policy
• Temporary and Permanent Fiscal Policy
• Limitations of Fiscal Policy?
• How Politics Affects Discretionary Fiscal Policy
• Government Borrowing
• National Savings and Investment Identity
• Debtor Nation
• Fiscal Policy Affects Trade Balances
• Twin Deficits
• Exchange Rates Affect Budget and Trade Deficits
• What are the risks of chronic large deficits in the United States?
• How Fiscal Policy Can Affect Trade Imbalances
• Government Borrowing Affect Private Savings
• Ricardian Equivalence
• Fiscal Policy Affects Investment and Economic Growth
• Crowding Out of Physical Capital Investment?
• How Does Government Borrowing Affect Interest Rates in Financial Markets?
• Government Investment in Physical Capital
• Public Investment in Human Capital
• Fiscal Policy Can Affect Technology Development

• Economic Cycle or Business Cycle
• Business Cycle Indicator
• Peak and Trough
• Recession and Depression
• Hard Landing vs Soft Landing
• Economic Bubble
• Boom and Bust Cycle
• Great Depression
• Baby Boomer Age Wave Theory

• Skyscrapper Effect (Economics)
• V-Shaped Recovery
• W-Shaped Recovery
• U-Shaped Recovery
• Kondratieff Wave Cycle
• Contagion
• Feedback Rule Policy
• American Customer Satisfaction Index
• CNN Effect
• Bureau of Economic Analysis
• Business Starts Index
• American Recover and Reinvestment Act
• Abenomics
• Emergency Economic Stabilization Act of 2008
• Commodity Credit Corporation
• Humphrey Hawkins Act

• Real Economic Growth Rate
• Fox-Trot Economy

• Stagnation
• Neoclassical Growth Theory
• Exogenous Growth Theory
• Endogenous Growth Theory
• New Growth Theory - Explained
• Classical Growth Theory - Explained
• Real Economic Growth Rate - Explained
• Plutonomy