Scientists believe they have discovered the gene that controls fear.
Researchers at the Howard Hughes Medical Institute at Columbia University in the United States found the gene, which produces a protein inhibiting the part of the brain that learns fear.
Two groups of mice - one lacking the protein receptor in the brain - were pitted against each other in tests which involved giving electric shocks. The normal mice and the receptor-lacking "knockout" group were trained to associate the sound of a tone with a shock.
After the training, the researchers compared the degree to which the two strains of mice showed fear when exposed to the same tone alone - by measuring the duration of a characteristic "freezing" response that the animals show when fearful.
"When we compared the mouse strains, we saw a powerful enhancement of learned fear in the knockout mice," Professor Eric Kandel, of the medical school, told the journal Cell.
The gene, however, appeared only to affect fear which could be learned, rather than that which is instinctive for animals.
Prof Kandel said: "While I don't want to overstate the case, in studies of fear learning we could well have an excellent beginning for animal models of a severe mental illness".
AP