Facial expressions: Blind (l) and sighted (r) relatives share similar facial expressions, even when separated at birth as in the top example.
COURTESY OF PNAS

A blind young man shares his mother's habit of compressing his lips together when puzzled, despite never having seen her face. This is just one of the examples cited as part of a study published online today in Proceedings of the National Academy of Sciences USA showing that relatives who have never seen one another nonetheless share similar facial expressions--proof that even a grimace may be hereditary.

Darwin was among the first to note that facial expressions are innate for humans and other animals. Research by Paul Ekman of the University of California, San Francisco, has shown that human facial expressions are universal; disgust is disgust whether you are in New York or New Guinea. Yet there are individual differences in such facial displays of emotion, governed by variations in facial muscles or nerves; a smile may be dimpled or not while still remaining a recognizable sign of happiness. And a human's quota of expressions emerges within the first six months of life and remain largely stable for the rest of it, scientists have found.

Although the universality of expressions and differences in facial musculature argue for a hereditary basis, only a few studies with twins have examined that hypothesis. Given that even twins may learn expressions by observing their counterparts, Gili Peleg of the University of Haifa in Israel and her colleagues tested the surmise in 21 families containing one blind relative. While videotaping the subjects--21 blind and 30 sighted relatives--the researchers asked them to solve difficult puzzles, relate a personal experience that connected to a sad or joyful emotion, listen to a disgusting story, and respond to a question in gibberish. All told the researchers examined six emotional states: concentration, anger, sadness, joy, disgust and surprise.

The blind subjects showed significantly similar expressions to their sighted relatives when concentrating, sad or angry compared with the same emotions reflected on the faces of nonrelatives. And a computer trained to recognize similar faces correctly linked blind and sighted relatives 80 percent of the time. Further, surveys revealed that the blind did not spend time touching their relatives' faces to memorize them (or perhaps copy their expressions). "In fact, they consider this 'Hollywood' myth very impolite behavior," the researchers write.

Even relatives separated at birth shared expressions: The blind young man mentioned above was abandoned by his mother two days after birth and not reunited with her until he was 18 years old, yet they shared at least three facial expressions, which reinforces what Darwin suspected more than 100 years ago. As he wrote in The Expression of the Emotions in Man and Animals: "The inheritance of most of our expressive actions explains the fact that those born blind bear them, as I hear from the Rev. R. H. Blair, equally well with those gifted with eyesight."