Eve: How the Female Body Drove 200 Million Years of Human Evolution

by Cat Bohannon

The majority of human women favor cradling their infants and nursing them from our left breast, which also happens to line our baby up with the side of our face that is more expressive. No, really—and other primates do this, too. Among humans, the muscles on the left side of the face are slightly more adept at social signaling, and 60 to 90 percent of women preferentially cradle infants toward the left of the body’s midline, with the baby’s head more exposed to the left side of her face. This preference is strongest in the infant’s first three months of life, precisely the period when new mothers are nursing more often throughout the day. This is true across many human cultures and historical periods.