Researchers read a baby's looking, listening and sucking, and about what those readings cannot say. One way to learn what a baby expects is to watch the baby's eyes. 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.
Looking studies depend on what babies can see. At birth, babies cannot see as well as older children or adults, because their eyes and visual system are not fully developed, but vision improves a lot during the first few months. Newborn babies like to look at faces, and they also like black-and-white or high-contrast patterns. A newborn's eyes focus best on things about 8 to 12 inches (20 to 30 centimeters) away, about as far as a parent's face. By eight weeks, babies begin to focus more easily on the faces of the people near them.
For the first two months a baby's eyes are not well coordinated and may seem to wander or look crossed, which is usually normal. By about 2 months many babies watch people as they move, and in the first months they start tracking moving objects with their eyes. Depth perception, the ability to judge whether objects are nearer or farther, is not present at birth, and around the fifth month the eyes can work together to see in three dimensions. Seeing takes both the eyes and the brain, because a baby's brain has to learn to use what the eyes send.
Reflexes are movements a baby makes without choosing to, and doctors check a newborn's reflexes to see whether the brain and nervous system are working well. When a baby's cheek is touched, the baby turns toward the touch and opens their mouth, which is called the rooting reflex. When a baby's palm is stroked, the baby's fingers close around a finger in what is called the grasp reflex. A loud sound or a sudden move can make a baby stretch out their arms and legs and then pull them back in, a response called the startle reflex.
The tonic neck reflex, also called the fencing pose, happens when a baby's head turns to one side: the arm on that side stretches out, and the other arm bends up. 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. A baby's toes curl when the sole of the foot is touched, a grasp reflex in the feet that lasts until about 9 to 12 months. Babies also have a hand-to-mouth reflex that goes with rooting and sucking, so they may suck on their fingers or hands.
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. Babies also begin to smile back at people who talk and smile to them, usually within the first few months. Around 4 months many babies smile on their own to get a grown-up's attention and chuckle, and around 6 months they laugh. 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. In the first few months, up to about 4 months, babies coo, making soft pleasure sounds like oooo and aahh.
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 the second half of the first year babies enjoy peek-a-boo, and by around 12 months they find hidden objects easily.
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. Although Piaget's finding that infants progress in how they search for hidden objects has been replicated, his deeper inference can be questioned. One study found that infants responded to two types of total hiding in different ways, which suggests object permanence is not a once-and-for-all attainment.
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). Babies can hear loud sounds from the start, and they may turn their head toward familiar voices, while newborns begin to recognize important sounds around them, such as the voice of a caregiver. 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 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. Newborns also prefer their native language over a foreign one, and babies of bilingual mothers respond equally to both of the mothers' languages. In a 2015 study, newborns showed an absent or weak sucking response to a brief recording of their mother saying baby, even though earlier brain recordings showed they could tell her voice from others. Even when a newborn's brain shows it recognizes a sound, there may be no measurable behavior, because the newborn may not be motivated enough to respond.
In the first few years of life the brain grows new connections at a very fast rate. As of November 2026, according to Singapore's Health Promotion Board, a baby's brain roughly doubles in weight in the first year. The brain is built from the bottom up, so simple connections and skills develop first, followed by more complex circuits and skills. After a period of especially rapid growth in the first few years, the brain refines itself through pruning, which makes its circuits more efficient. The early years are the most active time for making neural connections, but new connections can form throughout life. Genes provide the blueprint for the brain's circuits, but repeated use strengthens the circuits used most often, based on a child's experiences. The brain contains billions of connections between neurons, which allow lightning-fast communication among neurons that specialize in different jobs. The brain's ability to adapt, called plasticity, is greater in a child's earliest years and declines as we age.
As babies grow they begin to sort out the speech sounds that make up the words of their language. Babies learn the sounds of the language around them, and by 6 months most recognize its basic sounds, while by the end of the first year they have learned a great deal about its sound structure. Learning language is more than making sounds, because listening, understanding and knowing the names of people and things are all part of it. The first 3 years of life are the most intensive period for acquiring speech and language skills, and there appear to be critical periods when the brain is best able to absorb language. Naming what a young child is seeing, doing or feeling helps make language connections, even before the child can talk or understand words.
Studies using artificial languages suggest that infants use statistical information in the speech stream to find where words begin and end. 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. Researchers showed that babies could tell the artificial words from nonwords by measuring how long the babies listened in a head-turn preference procedure. 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.
Real baby-directed speech also has higher transitional probabilities within words than across word boundaries, like the artificial language. Seventeen-month-olds who first heard a segmentation task could then learn object labels that were words from that speech, but not labels made of novel or low-probability syllable sequences. Some scientists point out that the results show babies tell sequences with low and high internal transitional probabilities apart. Whether those sequences are truly wordlike is still an open question.
Words to know
- habituation: getting used
- transitional: between-step
- permanence: lasting existence
- replicated: repeated successfully
- reflexes: automatic movements
