Eyebrow flash

The eyebrow flash is an unconscious social signal, wherein a person, wishing to approach another whom they recognize and are preparing for social contact, raises their eyebrows for approximately one-fifth of a second. People generally return an eyebrow flash, unless it was given by someone whom they do not know, or someone who looks away immediately after. The message must be interpreted in context. Psychologists and sociologists say that eyebrow raising can be a reaction to fear or surprise.

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Wikipedia