Two studies found that 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” eyes appeared.
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.”