公的貯蓄と民間貯蓄

Private saving equals household income minus taxes and consumption expenses, expressed as S = Y − T − C. For example, a household earning $80,000 annually, paying $20,000 in taxes, and spending $50,000 on goods leaves $10,000 in private savings. This remaining income can be invested in retirement accounts, stocks, or savings accounts, contributing funds to financial markets for business investment.

A budget surplus occurs when government tax revenue exceeds spending, representing positive public savings. Conversely, a budget deficit occurs when government spending exceeds tax revenue, representing negative public savings. For instance, collecting $3 trillion in taxes while spending $2.8 trillion creates a $200 billion surplus, whereas spending $3.2 trillion creates a $200 billion deficit.

In a closed economy, national saving equals total investment through the national income identity. National saving is what remains after consumption and government spending: S = Y − C − G. Substituting the income equation Y = C + I + G yields S = I, proving total savings must equal total investment. This fundamental relationship ensures all available funds finance productive investment.

Private saving is influenced by interest rates, consumer confidence, and access to financial products. Tax incentives for retirement contributions, such as IRAs, also encourage higher private saving rates. When interest rates rise or consumer confidence strengthens, households tend to save more. Conversely, lower rates or reduced confidence may decrease savings as households prioritize current consumption.

During recessions, governments typically increase spending to stimulate demand, often resulting in budget deficits and negative public savings. During economic booms, governments may reduce spending or increase taxes to pay down debt or build reserves, creating surpluses. These fiscal policy decisions directly determine whether public saving contributes positively or negatively to national saving.

National saving comprises two components: private saving from households and public saving from government. Private saving represents household income after taxes and consumption. Public saving represents the difference between government tax revenue and spending. Together, these components form total national saving, which must equal investment in a closed economy and reflects the economy's capacity for productive investment.

In closed economies, national saving must equal investment. However, in open economies, differences between saving and investment reflect net capital flows across borders. When a nation saves more than it invests domestically, excess capital flows outward. Conversely, when investment exceeds domestic saving, capital flows inward. These capital flows influence trade balances and currency valuation in open economies.