Infants (birth to age 1) and toddlers (ages 1 to 2) grow quickly; bodily changes are rapid and profound. Physical development refers to biological changes that children undergo as they age. The first 4 weeks of life are termed the neonatal period. Most babies weigh between 2.5 and 4.5kg, and are between 45 and 55cm long. Male babies are generally slightly heavier and longer than female babies. Both premature and postmature babies are at higher risk of complications such as sickness, brain damage, or death, than are full-term babies. Physical growth is especially rapid during the first 2 years. An infant's birthweight generally doubles by 6 months and triples by the infant's first birthday. Similarly, a baby grows between 25 and 30cm in length (or height), and the baby's proportions change during the first 2 years. The size of an infant's head decreases in proportion from 1/3 of the entire body at birth, to 1/4 at age 2, to 1/8 by adulthood. Fetal and neonatal brain developments are also rapid. The lower, or subcortical, areas of the brain (responsible for basic life functions, like breathing) develop first, followed by the higher areas, or cortical areas (responsible for thinking and planning). Most brain changes occur prenatally and soon after birth. At birth, the neonate's brain weighs only 25% of that of an adult brain. By the end of the second year, the brain weighs about 80%; by puberty, it weighs nearly 100% of that of an adult brain. Because infants cannot endure on their own, newborns have specific built-in or prewired abilities for survival and adaptive purposes. Reflexes are automatic reactions to stimulation that enable infants to respond to the environment before any learning has taken place. For instance, babies automatically suck when presented with a nipple, turn their heads when a parent speaks, grasp at a finger that is pressed into their hand, and startle when exposed to loud noises. Some reflexes, such as blinking, are permanent. Others, such as grasping, disappear after several months and eventually become voluntary responses. The rates of physical and motor developments differ among children depending on a variety of factors, including heredity, the amount of activity the child participates in, and the amount of attention the child receives. The baby's first relationship is generally with family members, to whom the infant expresses a range of emotions (and vice versa). If the social and emotional bonding between infant and family is faulty in some way, the child may never develop the trust, self-control, or emotional reasoning necessary to function effectively in the world. The quality of the relationship between child and parents—especially when the child is between the ages of 6 and 18 months—seems to determine the quality of the child's later relationships. Children between 7 and 24 months of age experience separation anxiety, or distress at the prospect of being left alone in an unfamiliar place. Related to separation anxiety is stranger anxiety, or distress in the presence of unfamiliar people. Separation and stranger anxieties are strong indicators of the attachment process, as the child may now distinguish between familiar and unfamiliar stimuli. Central to early cognitive development is memory development. Babies begin forming long-term memories during the first 6 months. Infants may recognize and remember primary caretakers, as well as familiar surroundings. Early memory experiences help infants and toddlers to understand basic concepts and categories, all of which are central to more completely understanding the world around them. Observational learning (imitation) and operant conditioning (reinforcement) play important roles in the early acquisition of language. Children are reinforced to speak meaningfully and reasonably by imitating the language of their caregivers; in turn caregivers are prompted to respond meaningfully and reasonably to the children.
Ages 2 through 6 are the early...
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