Infancy and Toddlerhood
During the first 2 years, body size increases dramatically—faster than at any other time after birth. Body
fat is laid down quickly in the first 9 months, whereas muscle development is slow and gradual. The best
way to estimate a child’s physical maturity is by using skeletal age. Two growth patterns—cephalocaudal
and proximodistal trends—describe changes in the child’s body proportions.
At birth, the brain is nearer than any other physical structure to its adult size, and it continues to
develop at an astounding pace throughout infancy and toddlerhood. Neurons, or nerve cells, that store
and transmit information, develop and form an elaborate communication system in the brain. As
neurons form connections, stimulation becomes necessary for their survival. The cerebral cortex is the
largest, most complex brain structure—accounting for 85 percent of the brain’s weight, containing the
greatest number of neurons and synapses, and responsible for the unique intelligence of our species. At
birth, the two hemispheres of the cerebral cortex have already begun to specialize, a process called
lateralization. However, the brain is more plastic during the first few years than it will ever be again.
Animal research and natural experiments with children who were victims of deprived early
environments provide evidence for sensitive periods in brain development. Appropriate stimulation is
key to promoting experience-expectant brain growth—the young brain’s rapidly developing
organization, which depends on ordinary experiences. Experience-dependent brain growth, in contrast,
occurs throughout our lives as a result of specific learning experiences. Rapid brain growth means that
the organization of sleep and wakefulness changes substantially between birth and 2 years, but the
social environment also plays a role.
Physical growth, like other aspects of development, results from the continuous and complex interplay
between genetic and environmental factors. Heredity, nutrition, and emotional well-being all affect
early physical growth. Dietary diseases caused by malnutrition affect many children in developing
countries. If allowed to continue, body growth and brain development can be permanently stunted.
Breastfeeding provides many benefits to infants, especially for those in the developing world where
safe, nutritious alternatives are not widely available. Breastfeeding also helps protect against later
obesity. Babies who do not receive affection and stimulation may suffer from nonorganic failure to
thrive, which has symptoms resembling those of malnutrition but has no physical cause.
Babies come into the world with built-in learning capacities that permit them to profit from experience
immediately. Classical and operant conditioning, habituation and recovery, and imitation are all
important mechanisms through which infants learn about their physical and social worlds.
Like physical development, motor development follows the cephalocaudal and proximodistal trends.
Babies’ motor achievements have a powerful effect on their social relationships. According to the
dynamic systems theory of motor development, each new motor skill is a joint product of central
nervous system development, movement capacities of the body, the child’s goals, and environmental
supports for the skill. Cultural differences in infant-rearing practices affect the timing of motor
Perception changes remarkably over the first year of life. Hearing and vision undergo major advances
during the first 2 years as infants organize stimuli into complex patterns, improve their perception of
depth and objects, and combine information across sensory modalities. From extensive everyday
experience, babies gradually figure out how to use depth cues to detect the danger of falling. According
to Eleanor and James Gibson’s differentiation theory, perceptual development is a matter of detecting
invariant features in a constantly changing perceptual world.
According to Piaget, by acting directly on the environment, children move through four stages of
cognitive development in which psychological structures, or schemes, achieve a better fit with external
reality. The first stage, called the sensorimotor stage, spans the first two years of life and is divided into
six substages. In this stage, infants make strides in intentional behavior and understanding of object
permanence until, by the end of the second year, they become capable of mental representation, as
seen in their sudden solutions to sensorimotor problems, mastery of object permanence, deferred
imitation, and make-believe play. Recent research suggests that some infants display certain
understandings earlier than Piaget believed, raising questions about the accuracy of his account of
Information-processing theorists, using computer-like flowcharts to describe the human cognitive
system, focus on many aspects of thinking, from attention, memory, and categorization skills to complex
problem solving. With age, infants attend to more aspects of the environment and take information in
more rapidly. In the second year, as children become increasingly capable of intentional behavior,
attention to novelty declines and sustained attention improves. As infants get older, they remember
experiences longer and group stimuli into increasingly complex categories. Also, categorization shifts
from a perceptual to conceptual basis. Information processing has contributed greatly to our view of
young babies as sophisticated cognitive beings. However, its greatest drawback stems from its central
strength—by analyzing cognition into its components, information processing has had difficulty putting
them back together into a broad, comprehensive theory. Vygotsky believed that complex mental
activities have their origins in social interaction. Through joint activities with more mature members of
their society, children come to master activities and think in ways that have meaning in their culture.
Infant intelligence tests primarily measure perceptual and motor responses and predict later intelligence
poorly. Speed of habituation and recovery to visual stimuli, basic information-processing measures, are
better predictors of future performance. Home and child-care environments, as well as early
intervention for at-risk infants and toddlers, exert powerful influences on mental development.
As perception and cognition improve during infancy, they pave the way for an extraordinary human
achievement: language. The behaviorist perspective regards language development as entirely due to
environmental influences, whereas nativism assumes that children are prewired with an innate language
acquisition device to master the intricate rules of their language. The interactionist perspective
maintains that language development results from interactions between inner capacities and
environmental influences. Babies begin cooing around 2 months, followed by babbling, which gradually
reflects the sound and intonation patterns of the child’s language community. First words appear
around 12 months, and two-word utterances between 18 and 24 months. However, substantial
individual differences exist in the rate and style of early language progress. As toddlers learn words, they
may apply them too narrowly (underextension) or too broadly (overextension), in part because their
language comprehension develops ahead of their ability to produce language. Adults in many cultures
speak to young children using child-directed speech, a simplified form of language that is well-suited to
their learning needs. Deaf parents use a similar style of communication when signing to their deaf
babies. Conversational give-and-take between adults and toddlers is one of the best predictors of early
language development and academic success during the school years.
Emotional and Social Development
Although Freud’s psychoanalytic theory is no longer in the mainstream of human development research,
his emphasis on the importance of the parent–child relationship was accepted and elaborated on by
other theorists, notably Erik Erikson. Erikson believed that the psychological conflict of the first year of
life is basic trust versus mistrust, and that a healthy outcome depends on the quality of the parent–child
relationship. During toddlerhood, the conflict of autonomy versus shame and doubt is resolved
favorably when parents provide appropriate guidance and reasonable choices. If children emerge from
the first few years without sufficient trust and autonomy, the seeds are sown for adjustment problems.
All humans and other primates experience basic emotions—happiness, interest, surprise, fear, anger,
sadness, and disgust—that have an evolutionary history of promoting survival. Emotions play powerful
roles in organizing social relationships, exploration of the environment, and discovery of the self.
Cognitive and motor development, caregiver–infant communication, and cultural factors all affect the
development and expression of emotions. Infants’ emotional expressions are closely tied to their ability
to interpret the emotional cues of others. As toddlers become aware of the self as a separate, unique
individual, self-conscious emotions—guilt, shame, embarrassment, envy, and pride— appear. Toddlers
also begin to use emotional self-regulation strategies to manage their emotions. Rapid development of
the cerebral cortex, sensitive caregiving, and growth in representation and language contribute to the
development of effortful control, which is necessary for self-regulation.
Infants vary widely in temperament, including both reactivity (quickness and intensity of emotional
arousal, attention, and motor activity) and self-regulation (strategies for modifying reactivity). Research
findings have inspired a growing body of research on temperament, examining its stability, biological
roots, and interaction with child-rearing experiences. The goodness-of-fit model explains how
temperament and environment can together produce favorable outcomes when childrearing
practices match each child’s temperament while encouraging more adaptive functioning.
Attachment refers to the strong affectionate tie we have with special people in our lives that leads us to
feel pleasure when we interact with them and to be comforted by their nearness in times of stress. By
the second half of the first year, infants have become attached to familiar people who have responded
to their needs. Today, the ethological theory of attachment, which recognizes the infant’s emotional tie
to the caregiver as an evolved response that promotes survival, is the most widely accepted view. By the
end of the second year, children develop an enduring affectionate tie to the caregiver that serves as an
internal working model, a guide for future close relationships. Attachment security is influenced by
opportunity for attachment, quality of caregiving, infant characteristics, and family circumstances.
Babies form attachments to a variety of familiar people in addition to mothers—fathers, siblings,
grandparents, and professional caregivers. Mounting evidence indicates that continuity of caregiving is
the crucial factor that determines whether attachment security in early life is linked to later
Children can recover from an insecure attachment history if caregiving improves. During the first two
years, knowledge of the self as a separate, permanent identity emerges, beginning with self-
recognition—identification of the self as a physically unique being. Self-awareness is associated with the
beginnings of empathy—the ability to feel with another person. Self-awareness also contributes to
effortful control—the extent to which children can inhibit impulses, manage negative emotion, and
behave in socially acceptable ways. Self-control allows toddlers to become compliant and acquire the
ability to delay gratification.