89. Minimum perceived risk
Aside from a ceiling on maximum risk set by past experience, people also have a floor of minimum perceived risk based on the minimum probabilities that a human being can intuitively care about. According to cognitive psychology experiments, this probability is about 0.01%, or one in 10,00060. If an experiment or procedure has a one in 10,000 chance of going awry, even if the consequences would be catastrophic, there is a strong tendency to ignore it. This can be a problem if the risk is a daily event, imperceptible on a day-by-day basis, but over a period of years or decades, a disaster is all but assured.