Highly emotive incidents trigger the brain to release the hormone and neurotransmitter noradrenaline. This stimulates the amygdala – part of the brain involved with processing emotional reactions – to store memories in the hippocampus and other parts of the brain, says Dominique de Quervain, a neuroscientist at the University of Zurich in Switzerland.

Yet for some reason, recall of emotional events varies a great deal from person to person. So de Quervain wondered if common variations in a gene called ADRA2B, which codes for the noradrenaline receptor, could be responsible. Some 30 per cent of Caucasians and 12 per cent of Africans possess this variant, he says.