One channel of crowding out is a reduction in private investment that occurs because of an increase in government borrowing. If an increase in government spending and/or a decrease in tax revenues leads to a deficit that is financed by increased borrowing, then the borrowing can increase interest rates, leading to a reduction in private investment. There is some controversy in modern macroeconomics on the subject, as different schools of economic thought differ on how households and financial markets would react to more government borrowing under various circumstances.
The extent to which crowding out occurs depends on the economic situation. If the economy is at capacity or full employment, then the government suddenly increasing its budget deficit (e.g., via stimulus programs) could create competition with the private sector for scarce funds available for investment, resulting in an increase in interest rates and reduced private investment or consumption. Thus the effect of the stimulus is offset by the effect of crowding out. On the other hand, if the economy is below capacity and there is a surplus of funds available for investment, an increase in the government's deficit does not result in competition with the private sector. In this scenario, the stimulus program would be much more effective. In sum, changing the government's budget deficit has a stronger impact on GDP when the economy is below capacity. In the aftermath of the 2008 subprime mortgage crisis, the U.S. economy remained well below capacity and there was a large surplus of funds available for investment, so increasing the budget deficit put funds to use that would otherwise have been idle.[4]
The macroeconomic theory behind crowding out provides some useful intuition. What happens is that an increase in the demand for loanable funds by the government (e.g. due to a deficit) shifts the loanable funds demand curve rightwards and upwards, increasing the real interest rate. A higher real interest rate increases the opportunity cost of borrowing money, decreasing the amount of interest-sensitive expenditures such as investment and consumption. Thus, the government has "crowded out" investment.