The cortex, especially, shows gains in myelinization and formation of synapses, followed by synaptic pruning. Hand preference strengthens, a sign of greater brain lateralization. In addition, connections between different parts of the brain increase. These changes support improvements in a wide variety of physical and cognitive skills. Factors affecting physical growth and health in infancy and toddlerhood continue to be influential in early childhood.
Heredity affects physical growth by regulating the production of hormones.
Extreme emotional deprivation can interfere with the production of growth hormone, thereby stunting children's growth. Sleep difficulties, in the form of night waking and nightmares, are common during the preschool years. Appetite declines due to a slower rate of physical growth. Since caloric intake is reduced, preschoolers need a high-quality diet. Disease can lead to malnutrition, seriously undermining children's growth, an effect that is especially common in developing countries.