Psychologist John Leach, a specialist in human responses to emergency situations, developed his “10/80/10 rule of survival” after examining a variety of crises and human reactions to them. According to Leach, 10 percent of people facing an emergency control their fears and act rationally. Eighty percent find themselves stunned and relatively unprepared to respond. The last 10 percent, Leach concluded, become hysterical, unable to cope with the situation at hand. To ensure you land in that top 10 percent, you should first understand the physiological processes your brain and body undergo during an emergency.