The smell of fear, one of the most terrible cliches of pulp fiction, is actually founded in fact, scientists claim today.

People can unconsciously detect whether someone is stressed or scared by smelling a chemical pheromone released in their sweat, according to researchers who have investigated the underarm secretions of petrified skydivers.

The team found that the smell of fear triggered a heightened response in brain regions associated with fear when inhaled by volunteers in a brain scanner. The research suggests that, like many animal species, humans can detect and subconsciously respond to pheromones released by other people.

The research was funded by the US Defence Advanced Research Projects Agency - the Pentagon's military research wing - raising speculation that it is a first step to isolating the fear pheromone for use in warfare, perhaps to induce terror in enemy troops. But Darpa denied that it had any military plans for fear pheromones or plans to fund further research into the field.

Dr Lilianne Mujica-Parodi at Stony Brook University in New York State and her team taped absorbent pads to the armpits of 20 novice skydivers - 11 men and nine women - on their first tandem jump. The pads soaked up sweat before they leapt from the plane and as they fell. For comparison, the team collected sweat from the same individuals as they ran on a treadmill for a similar duration at the same time of day they had made their jump.

They transferred the two types of sweat to nebulisers and asked volunteers in a brain scanner to breath it in. The team did not tell the volunteers about the experiment. New Scientist magazine reported that the volunteers' amygdala and hypothalamus - brain regions associated with fear - were more active in people who breathed in the "fear" sweat. The volunteers in the brain scanner were unable consciously to distinguish between the two types of sweat.

In a conference presentation last year, Mujica-Parodi wrote: "We demonstrate here the first direct evidence for a human alarm pheromone ... our findings indicate that there may be a hidden biological component to human social dynamics, in which emotional stress is, quite literally, 'contagious'." She declined to comment further on the results because the study is under review with a scientific journal.

Simon Wessely, a psychiatrist at the King Centre for Military Health Research at King's College London, told New Scientist that the idea that a fear pheromone could be developed as a chemical weapon was scientifically implausible. He said that a purely physiological cue was not enough to induce fear.

Most researchers do not believe that humans can detect pheromones. In other mammals this is done using a structure in the nose called the vomeronasal organ. Although humans have one of these it is not connected to the brain. However, human pheromones could still be detected elsewhere and some small studies have suggested that human behaviour can be modified by an alarm pheromone.