Right hemisphere dominance for the production of facial expression in monkeys.

In humans, the left side of the face (right hemisphere of the brain) is dominant in emotional expression. In rhesus monkeys, the left side of the face begins to display facial expression earlier than the right side and is more expressive. Humans perceive rhesus chimeras created by pairing the left half of the face with its mirror-reversed duplicate as more expressive than chimeras created by right-right pairings. That the right hemisphere determines facial expression, and the left hemisphere processes species-typical vocal signals, suggests that human and nonhuman primates exhibit the same pattern of brain asymmetry for communication.