At seven o’clock in the evening of 27 September 1994, the cruise ferry MS Estonia left Tallin with 989 people on board, heading for Stockholm through the Baltic Sea. It never got there. Six hours into the journey, pushing through a force nine gale, the bow door broke open and the ferry started taking on water. Within an hour it had sunk, taking with it 852 of its passengers and crew.

Even given the speed of tragedy, the stormy sea and the length of time it took rescuers to arrive (a full-scale emergency was only declared half an hour after the sinking), survival experts were astonished at the high death toll. It appears that many people drowned because they did nothing to save themselves. “A number of people… seem to have been incapable of rational thought or behaviour because of their fear,” concluded the official report into the accident. “Others appeared petrified and could not be forced to move. Some panicking, apathetic and shocked people were beyond reach and did not react when other passengers tried to guide them, not even when they used force or shouted at them.”

What happened? One person who knows the answer is John Leach, a military survival instructor who researches behaviour in extreme environments at the University of Portsmouth. He has studied the actions of survivors and victims from dozens of disasters around the world over several decades (and as it happens he was present at one of them, the fire at King’s Cross underground station on 18 November 1987 which killed 31 people). He has found that in life-threatening situations, around 75% of people are so bewildered by the situation that they are unable to think clearly or plot their escape. They become mentally paralysed. Just 15% of people on average manage to remain calm and rational enough to make decisions that could save their lives. (The remaining 10% are plain dangerous: they freak out and hinder the survival chances of everyone else.)