The normalcy bias, or normality bias, is a belief people hold when considering the possibility of a disaster. It causes people to underestimate both the likelihood of a disaster and its possible effects, because people believe that things will always function the way things normally have functioned.
On 911, individuals who chose to evacuate immediately upon command, fled the tower stairwells and lived. Individuals who chose to not heed the command and return to their desk to shut down their computers and grab their personal belongings, perished.