Monkeys disclose the face of emotion.

Smiles, frowns, and other emotional expressions attain greater intensity on the left side of the face, at least among right-handed adults. Scientists argue that the brain's right hemisphere, which controls many muscles on the left side of the face and largely regulates emotional displays, produces these slightly lopsided looks (SN: 3/11/89, p. 149).

Evidence now suggests that rhesus monkeys display the same left-sided bias in facial expressions. This finding, combined with previous data suggesting that the left hemisphere regulates language in humans and vocal signals in monkeys, indicates that division of communication functions between the left and right brain is similar in all primates, asserts Marc D. Hauser, an anthropologist at Harvard University 

In a frame-by-frame analysis of videotaped fear grimaces made by 19 free-ranging rhesus monkeys, Hauser found that a large majority of the animals moved the left side Of the mouth first and retracted the lips further on the left side while grimacing. Mouth and ear movements used as threats showed the same left-sided emphasis, as did grimaces during copulation, Hauser reports in the July 23 SCIENCE.

Of 43 human adults who viewed two composite images of a monkey's fear grimace - in which each side of the face was paired with its mirror image -- nex t to the creature's genuine fear grimace, 41 rated the left-side composite as most expressive of fear, Hauser notes.