Abstract: In explaining how public fear might produce unjustified intrusions on civil liberties, emphasizes two potential sources of error: the availability heuristic and probability neglect. The availability heuristic, widely used by ordinary people, can lead to a grossly exaggerated sense of risk, as salient incidents make citizens think that a risk is far more serious than it actually is. When probability neglect is at work, people focus on the "worst case" and disregard the question whether it is likely that the worst case will occur - an approach that can also lead to excessive regulation. With an understanding of the availability heuristic and probability neglect, believes it possible to have a better appreciation of the sources of unsupportable intrusions on civil liberties.