Fat increases faster than muscle, and the skull expands rapidly to accommodate the rapidly growing brain. Neurons form intricate connections and their fibers myelinate, causing a rapid increase in brain weight. Already, the two hemispheres of the cortex have begun to specialize, although the brain retains considerable plasticity during the first few years. In addition, specific sensitive periods in brain development are now recognized, when the brain must receive appropriate stimulation for it to reach its full potential. A variety of factors affect early physical growth.
Heredity contributes to height, weight, and rate of physical maturation.
Nutrition is essential for rapidly growing babies, and breast milk is especially suited to meet their needs. Malnutrition during the early years can result in permanent stunting of physical growth and of brain development. Affection and stimulation are also essential for healthy physical growth. Infants are marvelously equipped to learn immediately after birth. Classical conditioning, operant conditioning, habituation-dishabituation, and imitation are important early learning capacities that assist infants in finding out about their physical and social worlds.