So that's what eyebrows are for...

Why do we have a line of hair sitting caterpillar-like above each of our eyes? Forget such mundanities as dust protection or ornamentation, a new study suggests our eyebrows serve to control how easily other people can tell where we are looking.

Four participants looked at hundreds of photos of the eye region of people who were facing forwards, gazing either to the left or right of the camera. The photos varied in size (a substitute for what, in real life, would be viewing distance), how long they were shown for and whether the eyebrows were neutral, lowered or raised.

The participants' accuracy in judging gaze direction from the photos was unaffected by photograph size up to a critical point after which accuracy suddenly plummeted. Roger Wyatt and colleagues at Stirling University who conducted the research said this pattern of results suggests we determine a person's gaze direction by comparing the amount of white eyeball visible on either side of the dark iris.

Crucially, the position of the eyebrows affected the size of photo beyond which accuracy was affected. In other words, if the eyebrows were raised, the participants were able to judge gaze direction with smaller photos than if the eyebrows were lowered. Viewing time affected accuracy regardless of photo size.

The researchers said the effect of eyebrow position was suggestive of a social function for the hairy tufts, and helped explain the 'eyebrow flash' – the convention for people to raise their eyebrows at each other from a distance in a gesture of acknowledgement. “It may make the eye gaze direction of the sender briefly visible to the receiver. Since that eye gaze direction is at the receiver, it results in the receiver being given a brief 'I am looking at you signal', which would communicate recognition,” they said.
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Watt, R., Craven, B. & Quinn, S. (In Press). A role for eyebrows in regulating the visibility of eye gaze direction. The Quarterly Journal of Experimental Social Psychology.

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