The phrase "don't be such a baby" implies that expressing fear is childish. According to Abigail Marsh, a Harvard Ph.D. and researcher at the National Institute of Mental Health, this may be what nature intended.

In a paper to be published in January's Personality and Social Psychology Bulletin, Marsh reports that frightened faces--with their wide, round eyes, raised eyebrows and rounded lower region--are perceived to resemble a baby's facial appearance. In one study, participants rated images of fearful and angry faces. Fearful faces were judged less mature, weaker, more submissive and more babyish. In a second study, Marsh created composite images that made it difficult to read the posers' emotional expression. She superimposed the eyes, eyebrows and nose bridge from a person's fear or anger expression onto an image of the same person's happy expression. The fearful composites were still judged as less mature and more babyish when compared with the angry ones.

The implication is that the physical configuration of a fearful face doesn't just convey the fear itself, but may help protect the frightened person. "The 'fear' facial expression may have evolved to mimic the appearance of a baby's face in order to elicit similarly caring, nonaggressive behavior from people who see it," says Marsh. That's welcome news for babies and adults alike.