31.1 How Government Borrowing Affects Investment and the Trade Balance

National Saving and Investment

National saving and investment identity

The national saving and investment identity connects a country's output to how it's spent and financed. It's the foundation for understanding why government borrowing affects private investment and trade.

Start with the GDP equation:

$Y = C + I + G + NX$

• $Y$ = national income (GDP), the total value of domestically produced goods and services
• $C$ = consumption spending by households
• $I$ = investment spending by businesses on capital goods (machinery, equipment, structures)
• $G$ = government purchases of goods and services (this excludes transfer payments like Social Security)
• $NX$ = net exports (exports minus imports)

You can rearrange this equation to isolate saving. National saving ($S$) is the portion of income not consumed by households or used for government purchases:

$S = Y - C - G$

National saving has two parts:

• Private saving: what households keep after taxes and consumption, or $(Y - T) - C$
• Public saving: what the government collects in taxes minus what it spends, or $T - G$

In a closed economy (no international trade), there's nowhere for saving to go except domestic investment, so $S = I$. Every dollar saved finances a dollar of investment.

In an open economy, capital can flow across borders, so saving and investment don't have to match:

$S = I + NX$

This identity tells you something powerful:

Government Budget Deficits and Their Impact

Impact of government budget deficits

A government budget deficit occurs when government spending exceeds tax revenue, meaning public saving is negative. Since national saving = private saving + public saving, a budget deficit directly reduces national saving (assuming private saving doesn't rise enough to offset it).

That reduced saving has to show up somewhere. The identity $S = I + NX$ tells you there are only two possible outcomes:

1. Domestic investment falls (crowding out). The government borrows from the same pool of loanable funds that businesses use. This increased demand for funds pushes interest rates up, making it more expensive for firms to borrow and invest.
2. The trade deficit grows. Higher interest rates attract foreign investors seeking better returns. Foreign capital flows in, which increases demand for the domestic currency, causing it to appreciate. A stronger currency makes exports more expensive and imports cheaper, widening the trade deficit.

In practice, both effects typically happen at the same time. The twin deficits hypothesis captures this pattern: budget deficits tend to produce trade deficits because reduced national saving forces the country to borrow from abroad to maintain its level of investment.

Trade surpluses and deficits in the national saving equation

The open economy identity $S = I + NX$ makes the link between saving and trade concrete.

Trade surplus ($NX > 0$): The country produces more than it consumes and invests domestically. The surplus saving gets channeled abroad as loans or foreign asset purchases. Countries like Germany and China have historically run large trade surpluses, reflecting high national saving rates relative to domestic investment.

Trade deficit ($NX < 0$): The country spends more than it produces, financing the gap by borrowing from foreign lenders. The U.S. has run persistent trade deficits for decades, partly reflecting low national saving driven by large government budget deficits.

Trade balances also shape which sectors of the economy grow:

• Countries with trade surpluses tend to have larger export-oriented sectors (like manufacturing)
• Countries with trade deficits tend to see growth in domestically focused sectors (like services)

The key takeaway is that trade deficits aren't just about imports and exports. They reflect a deeper imbalance between what a country saves and what it invests. Government borrowing is one of the most direct ways that imbalance gets created.