When babies are five months old, they can distinguish among faces of all races equally well. Past studies show they can, for instance, match a happy sound with many kinds of happy faces with equal ease. Yet by nine months, babies react more swiftly to their own race than others: they differentiate more readily between faces and match emotional sounds with facial expressions faster. A study from the University of Massachusetts Amherst, published in May in Developmental Science, showed that the younger infants use only the frontal part of the brain for the task. By nine months, babies also recruit the occipital-temporal region, where recognition happens in adults.

“The older babies are tending to use more of these adultlike face-processing, object-recognition regions of the brain,” says psychologist Lisa Scott, an author of the study. “Their brains weren't trying as hard,” she notes, because the older infants have more experiences to draw on. This finding adds to the theory that the newborn brain weighs most inputs and stimuli equally, perhaps resulting in the mingling of the senses known as synesthesia. As the brain matures, it learns to attend more to the sights and sounds important in the baby's life, such as faces that match his or her caregivers' race and sounds in the infant's native language. This theory matches physical neural development: a newborn's brain is massively connected, and over time important circuits are strengthened and unnecessary wiring is pruned.