1) Government spending multipliers are generally small when the economy is not at the zero lower bound (ZLB), and are increased only slightly by higher uncertainty. 2) When the economy is at the ZLB due to large negative shocks to bond returns or discount rates, multipliers can be substantially above one and increase further with higher spending or productivity volatility. 3) Different types of shocks can drive the economy to the ZLB, but bond return and monetary policy shocks are the primary drivers, while productivity shocks yield relatively small multipliers.