The psychological effects of terrorism make it hard to apply an economic cost-benefit analysis. While terrorism ranks low as a source of risk, the people regard it as a major danger—public tolerance for terrorism is near zero.

Many criticize security as being “just for show.” However, illusion is an important component of security. The objective is to convince would-be attackers that they will fail. We tend to focus on detection and prevention. Judging by the evidence, the most important effect of security is deterrence. There are very few instances where terrorists are caught trying to smuggle weapons or bombs on board airliners. If deterrence is working, that means fewer attempts, but it is difficult to count things that don’t occur.

Teams that test security measures by trying to get past those measures could add artificial events to the universe of terrorist attacks. However, those teams test only detection, not deterrence.

While quantifiable preventions of terrorist attacks by physical security measures are rare, we do have indirect indicators of their effects. Aviation security, the most ambitious security effort, offers several examples. Airline security measures have increased over the last four decades since 100 percent passenger screening was imposed in response to the increase in hijackings during the late 1960s and early 1970s.