During World War II and the Korean War, real GDP grew by about half the increase in government purchases. With allowance for other factors holding back GDP growth during those wars, the multiplier linking government purchases to GDP may be in the range of 0.7 to 1.0, a range generally supported by research based on vector autoregressions that control for other determinants, but higher values are not ruled out. New Keynesian macroeconomic models yield multipliers in that range as well. Neoclassical models produce much lower multipliers, because they predict that consumption falls when government purchases rise. Models that deliver higher multipliers feature a decline in the markup ratio of price over cost when output rises, and an elastic response of employment to increased demand. These characteristics are complementary to another Keynesian feature, the linkage of consumption to current income. The GDP multiplier is higher—perhaps around 1.7—when the nominal interest rate is at its lower bound of zero.