When presented with images of terrifying events, people tend to miscalculate their probability. A single memorable image -- of the World Trade Center collapsing, for example -- will crowd out less visually dramatic risks in the public mind. This explains why people overestimate the frequency of deaths from disasters like floods and fire and underestimate the frequency of deaths from more mundane threats like diabetes and strokes.
How can we protect ourselves from our psychological vulnerabilities? First, we can turn off the TV. A study of psychological responses to 9/11 found that, two months after the attacks, 17 percent of the American population outside New York City reported symptoms of post-traumatic stress related to 9/11. High levels of stress were especially notable in those who watched a lot of television. This anxiety is only heightened by cable networks, which have converted themselves into 24-hour purveyors of alarm.