For a resilient and committed nation, these bombings appear to have little residual economic effect. This should be unsurprising, because even well-orchestrated military bombing campaigns take considerable time to slow economic activity. German military production actually peaked more than two years after the start of our heavy bombing campaign during World War II.
There is very little evidence that terror bombings elsewhere have had much lasting effect beyond the immediate though painful loss of life and property. Moreover, these sorts of terror campaigns rarely have commendable results for antagonists, no matter the merit of their cause. For example, the success of the African National Congress was conditioned on its moral stance, not its bombings. The more than 10,000 IRA bombings have done nothing to thwart the wishes of actual voters in Northern Ireland.
Still, the terror bombing in Boston was clearly designed to scare us. Targeting sporting events, malls and the like are intended to make us alter our daily lives. In places very much under the cloud of war, such as Iraq, this may prevent a legitimate government from forming and taking control. Our enemies cannot believe that will happen here. Instead, what they want is for us to take extreme and costly steps to stop these attacks. From our fellow citizens we will hear calls to change our ways. Some will say we should spend so much on security so no one ever dies from a terror attack. Others will say we should carefully watch foreigners in our midst, while others will say that we should withdraw from the world and worry only about ourselves. We should listen respectfully and dismiss these options. We mustn't change ourselves for them.