It is hardly irrational to be scared in the immediate aftermath of an attack, when people may not know the scope of the threat and the extent of the danger. An elevated sense of vulnerability is hard to avoid. Yet even when individuals are highly unlikely to be at risk, they might remain fearful, simply because the horrible outcome is so vivid.
Nonetheless, it is possible for people to have a realistic sense of what risk is and what it isn't. Going about your business can be a good way to reduce individual, social and economic harm - and it can be a forceful answer to those who seek to frighten us.
Cass R. Sunstein, the Robert Walmsley University Professor at Harvard Law School, is a Bloomberg View columnist. He is the former administrator of the White House Office of Information and Regulatory Affairs, the co-author of "Nudge" and author of "Simpler: The Future of Government," just published by Simon and Shuster.