A primeval, and sometimes ugly, survival instinct swept over some of the desperate passengers of Air France Flight 358 when they found themselves trapped in the burning plane that skidded off a Toronto runway Tuesday.
It was for a time, as several passengers described it, everyone for himself or herself.
Stephanie Paquin, a 17-year-old returning from a student exchange in France, said "people were just pushing. They didn't care about anyone else." After fleeing through the emergency exits, "everyone was trampling everyone."
Johnny Abedrabbo, a 32-year-old economist, was also struck by the "pushing and shoving" as the first wave of panic set in among his fellow passengers.
Not everyone turned aggressive in their bid to flee the danger, however. Some were helpful, such as the stranger who pulled Ms. Paquin out of the ravine where the plane ended its slide.
But before the plane's emergency exits popped open, the range of emotions that played out in the crowded aisle of the Airbus A-340 was a primitive one.
Psychologists who study human behaviour under traumatic conditions say these involuntary survival responses have evolved with us from the earliest days of life on the planet.
"We have a bunch of primitive reflexes in which we exhibit behaviour like animals in certain situations," said Steven Taylor, a professor of psychiatry at the University of British Columbia, whose research focuses on trauma and anxiety.
As Neil Rector, psychologist and head of the anxiety-disorders clinic at Toronto's Centre for Addiction and Mental Health, put it, "We are hard-wired to seek survival."
Generally, Dr. Taylor said, a person can experience four states of response to trauma or a progression of these states.
The first is a mode of hypervigilance, where "like a rabbit being hunted by a dog and hiding in the bush," a person remains very quiet and still, absorbing visual cues and scanning the perimeter for an escape route.
The second stage involves the urge to flee, followed by the instinct to physically overcome whatever obstacles prevent the escape, commonly known as the fight-or-flight response.
It is during this third state that people can seem to lose their sense of compassion, Dr. Taylor said, noting this may help explain temporarily trapped passengers who push and shove as panic sets in. Often people will "later feel embarrassed" for their behaviour.
But Dr. Rector pointed out these are automatic reactions. One feature of the fight-or-flight response "is to become myopically focused on the threat at the expense of other elements of the environment. . . . The instinct for survival overrides the hierarchy of social niceties."
The fourth stage, Dr. Taylor said, is "tonic immobility," where a person might seem to freeze up and later describe the experience saying, " 'I couldn't move' . . . They would be just standing there until they were grabbed by someone saying, 'Come on, we've got to get out of here.' "
To some extent, Dr. Taylor said, the brain shuts itself down during trauma as a protective mechanism, or narrows the ability to focus. This may help explain why some passengers decided to stop to collect their carry-on luggage before fleeing the burning aircraft.
Dr. Rector noted that those passengers who collected their bags are also examples of the fact that not everyone perceives danger in the same way. "Some people perceived less of a threat, and so felt there was more time to leave in an orderly way."
Ms. Paquin, who stopped to gather her bags, was among those who did not realize the gravity of the situation. The Woodbridge, Ont., student was surprised by the shouting and panicking of others.
Not until she was off the plane, seeing it blazing and turned on its side, did she think, "Oh my God, I didn't know it was that serious at all."