Coming face-to-face with someone who looks scared triggers an automatic response in the brain that tells you to be afraid too, US researchers suggest.
Jenny McCarthy hiding in the movie Scream Three
Two studies found we respond to even the most subliminal danger messages.
One, in Science, found seeing the whites of the eyes triggered a danger message in the brain.
A second, in Neuron, showed that, even if an image of a scared face is shown too briefly to be consciously recognised, the brain registers it.
The first study, carried out by a team from the University of Wisconsin in Madison, looked at how the brain sensed danger.
It was known that the message was received in an almond-shaped part of the brain called the amygdala, but not what triggered that response.
The researchers scanned the brains of volunteers as they were shown a series of faces.
Before each face appeared, an image of either wide "fearful" eyes or smaller "happy"
And even though the images appeared so quickly that the volunteers were unaware of them, the scans showed that the wide eyes activated the amygdala.
But negative images, showing black eyes with white pupils, did not provoke the same response.
They said this meant that it was the sight of the whites of the eyes, known as the "sclera", triggers the response in the amygdala.
Writing in Science, the researchers led by Dr Paul Whalen, said: "Responsivity to eye whites, but not to eye blacks, appears to be driven by the size of the white scleral field and not by the outline of the eye."
They added: "Facial expressions are complex stimuli.
"Although there are holistic messages to be discerned - ie that person is afraid of something - this demonstration offers one example of a simpler rule that a subset of neuronal systems could use to prime additional circuits that will decode more detailed facial information and response systems."
The second study, carried out by researchers at Columbia University in New York, agreed that the amygdala - along with the attention and vision regions of the cerebral cortex - processed unconsciously detected threats.
They also looked at how a person's anxiety levels affected how they responded to potential threats.
The brains of 17 university students were scanned using high-resolution fMRI (functional magnetic resonance imaging) as they looked at a computer screen.
A fearful face was displayed for 33 milliseconds, immediately followed by a similar neutral face. The images flashed up too quickly to be consciously registered.
The students had normal variations in anxiety. But the more anxious a student was naturally, the more of a reaction was seen in the brain to a frightened face, the researchers found.
But when the students looked at the scared faces long enough for conscious recognition, a different brain circuit was used. That activity did not vary according to the underlying level of anxiety.
The researchers say their findings suggest more anxious people respond differently to fear triggers.
Dr Amit Etkin, who led the study, said: "Our study shows that there's a very important role for unconscious emotions in anxiety."
Joy Hirsch, who also worked on the study, added: "Psychologists have suggested that people with anxiety disorders are very sensitive to subliminal threats and are
picking up stimuli the rest of us do not perceive.
"Our findings now demonstrate a biological basis for that unconscious emotional vigilance."
She said the findings may mean it will one day be possible to use fMRI imaging to test new drugs to treat anxiety and to check a patient's response to therapy or
Iain Hutchison, consultant facial surgeon at Barts and the London NHS Trust, said: "The things that we look at first in people are their face, and usually the first facial feature we look at is their eyes.
"They convey subliminal messages that we pick up on."