In humans, the left side of the face (right hemisphere of the brain) i
s dominant in emotional expression. In rhesus monkeys, the left side o
f the face begins to display facial expression earlier than the right
side and is more expressive. Humans perceive rhesus chimeras created b
y pairing the left half of the face with its mirror-reversed duplicate
as more expressive than chimeras created by right-right pairings. Tha
t the right hemisphere determines facial expression, and the left hemi
sphere processes species-typical vocal signals, suggests that human an
d nonhuman primates exhibit the same pattern of brain asymmetry for co
mmunication.