Which facial expression is used as a sign of a greeting, interest or recognition?

An eyebrow flash is a form of non-verbal communication that is typically used as either a conscious or subconscious sign of interest or recognition.

This expression is very quick, often lasting only a fraction of a second, yet it can be recognized and returned by someone who is not even aware of it happening. An eyebrow flash typically involves the raising of both eyebrows quickly, before lowering them again to a comfortable position. It is often used between people who know each other as a quick but recognizable indication of familiarity, and it can be a sign of flirtation or interest.

A study conducted in 1987 recorded approximately 67 hours of unstaged social interactions conducted by 255 individuals from three locations: Western New Guinea, Papua New Guinea, and Upper Orinoco. The study found significant differences in the age and sex of senders and receivers among the populations studied; for instance, Eipo men from Western New Guinea were more likely to be senders, while Trobriand men from Papua New Guinea were more likely to be receivers. Across all three cultures, a lowering of the eyelids or a head movement often coincided with the beginning of an eyebrow flash. Additionally, people from all three cultures exhibited longer-duration eyebrow flashes at the openings of conversations than during conversations.