Emotion usually leads to an expression, but studies have shown that the process can also work in reverse: If you force your face to look sad or angry, the rest of your body will react as well, and you may involuntarily begin to feel those emotions. A look of anger will make your heart speed up and your blood vessels dilate until your skin turns red; a look of fear can make your hands cold and clammy and your hairs stand on end; a look of disgust can make you nauseated.

These internal responses may last a full minute or more—far longer than the expressions themselves, which last no more than two or three seconds. When people try to hide their emotions, their expressions may flash for one-fifteenth to one-twentieth of a second—just long enough for others to see them. After that, people can wipe away their “microexpressions,” as Ekman describes them. It is much harder to remove all traces of feeling from the voice. That’s why Ekman tells police to keep their suspects talking.

Thus, chimpanzees, like humans and some other non-human primates, show a right hemisphere specialization for facial expression of emotions, which suggests that this functional asymmetry is homologous in all these species.

ABOUT FACE: Eve Ekman, the daughter of psychologist Paul Ekman, is an accomplished facial mimic like her father. Here she demonstrates the seven major categories of facial expressions. Top row, from left to right: Fear, anger, happiness, contempt. Bottom row: surprise, disgust, and sadness. True anger is difficult to fake because most people cannot narrow the outer margins of their lips properly. True fear is hard because the eyebrows must be raised and drawn together simultaneously.