Babies can hear loud sounds from the start, and they may turn their head toward familiar voices. Newborns begin to recognize important sounds around them, such as the voice of a caregiver. Researchers showed that newborns would change how they sucked to switch on a recording of their mother reading a nursery rhyme (DeCasper and Fifer, 1980). In an exposure study, mothers read a nursery rhyme out loud during pregnancy, and their babies later preferred it, which shows learning before birth affects preferences after birth. The newborn brain also responds differently to even a brief snippet of the mother's voice and a stranger's, shown by scalp recordings (ERPs) in sleeping newborns. A newborn experiment with fathers' voices and other male voices showed newborns could tell the voices apart but did not show a preference.

Babies cry to tell the people near them what they need, such as food, a hug, a clean diaper or sleep. A baby's first signs of communication come when the baby learns that a cry brings food, comfort and company. A familiar, gentle voice can calm a crying baby. Babies cry more in their first 3 months than at any other time, and crying usually gets better after that. Serve and return is a back-and-forth between a baby and a caring grown-up, like a game of tennis. A baby coos or babbles, the adult answers, and the baby learns. Newborns also prefer their native language over a foreign one. Babies of bilingual mothers respond equally to both of the mothers' languages. As babies grow they begin to sort out the speech sounds that make up the words of their language. By 6 months most recognize its basic sounds.

Reflexes are movements a baby makes without choosing to. When a baby's cheek is touched, the baby turns toward the touch and opens their mouth, which is the rooting reflex. When a baby's palm is stroked, the baby's fingers close around a finger, which is the grasp reflex. The tonic neck reflex is also called the fencing pose: when a baby's head turns to one side, the arm on that side stretches out and the other arm bends up. A loud sound or a sudden move can make a baby stretch out their arms and legs and then pull them back in, which is the startle reflex. Doctors check a newborn's reflexes to see whether the brain and nervous system are working well. The grasp reflex lasts until a baby is about 5 to 6 months old, and then it fades away, while the rooting reflex fades at about 4 to 6 months, within the first several months of life.

Studies using artificial languages suggest that infants use statistical information in the speech stream to find where words begin and end, and segmentation here means finding where words begin and end in speech. In a famous 1996 study, infants heard about 2 minutes of an artificial language and were then able to pick out the words in the stream of speech. In that language the only clue to where words ended was statistics: the chance that one syllable follows another was 1.0 inside words and .33 across word boundaries. Researchers showed that babies could tell the artificial words from nonwords by measuring how long the babies listened in a head-turn preference procedure. Real baby-directed speech also has higher transitional probabilities within words than across word boundaries, like the artificial language.

In violation-of-expectation studies, infants see two test events. One matches what they expect, and the other breaks the expectation. With the right controls, reliably longer looking at the unexpected event than the expected event is evidence that infants hold the expectation, notice the violation and are surprised by it. Before the test events, infants may see habituation events that get them used to the test situation. In one study, habituation continued until a baby's looking time dropped by 50 percent or more on 3 trials in a row, compared with the first 3 trials, or until 9 habituation trials were done. The psychologist Jean Piaget found that babies of 5 to 7 months did not search for a toy hidden under a cloth, even though they could lift a cloth and grasp a toy. For several decades, researchers accepted Piaget's conclusion that young infants' mental pictures include only the objects they can directly perceive.

In experiments by Baillargeon and colleagues, 2.5-month-old infants were habituated to a toy mouse that moved back and forth along a track whose center was hidden by a screen. Infants begin to pay attention to height when objects go behind screens at about 3.5 months, and expect short but not tall objects to be hidden behind short screens. Looking-time results have been interpreted as showing that infants as young as 3.5 to 5 months, and perhaps from birth, show object permanence. That is a different conclusion from the one researchers accepted for several decades, that young infants' mental pictures include only the objects they can directly perceive.

Words to know

• newborns: brand-new babies
• segmentation: splitting into parts
• permanence: lasting existence
• habituation: getting used