The development of an individual is a process of physical, intellectual, and emotional growth and change. Two main factors heredity and environment determine a child’s development from birth onward.
The most rapid period of physical growth occurs during the first two years. Intellectual development is equally rapid during this time. In the first 12 months, the average infant’s weight triples and its body size increases by about half. As a general rule, by the age of two a baby’s height is already about half what it will be as an adult. The brain also develops rapidly and brain growth is largely complete by the age of two years. Motor skills and learning abilities improve as nerve tracts acquire their outer coating of myelin and develop new pathways between cells. These two processes continue up to about the age of five. The cartilaginous elements of the fetal skeleton also gradually harden into bone. Between the ages of five and seven, the first permanent teeth appear, and the bones of trunk and limbs continue to lengthen. The growth rate becomes less perceptible after about the age of six but speeds up again just before puberty.
During adolescence a person develops from a child to an adult a process that involves profound physical and emotional changes. Puberty the onset of sexual maturation occurs when, under the stimulation of the pituitary gland, the sex glands begin to release their hormones into the bloodstream. As a result, the reproductive organs mature and secondary sexual characteristics develop. Long bones undergo a burst of growth before attaining their final adult size. Muscles, brain, and central nervous system also complete their physical development at around this time, although some of the bones of the skull do not fuse permanently until the late thirties.
The early twenties mark the peak of physical development as muscles, heart, blood, and lungs all operate at maximum efficiency. Emphasis switches then from growth to maintenance and repair of the body. In the late twenties, however, the body begins its gradual physical decline. This process is hardly noticeable at first and in active people may not become apparent for many years. In most people, however, it becomes increasingly evident in their thirties and forties.
Exactly what causes aging is still uncertain, although genetic factors and wear and tear all play a part. The symptoms of old age are essentially an accumulation of defects: the nervous system, muscles, and skin deteriorate, major organs become less efficient, and disorders such as arteriosclerosis become more common.
Mental and social development
Mental and social development depend both on physical factors and on learning. According to the Swiss psychologist, Jean Piaget, intellectual development also moves through regular and recognizable stages. Until about the age of two, sensation and reflexes are closely linked. Gradually, the use of language develops and, as hand-eye and eye-ear coordination increase, mental processes develop from intuition based on incomplete perception to reasoned thought. By about the age of 12, a child has usually developed an adult ability to conceptualize and reason.
Less measurable, perhaps, is the process of social development, or socialization the acquisition of a “social self” with values and beliefs that influence an individual’s personal behavior and aspirations. This process begins in childhood, with the family being the chief socializing agent in most cultures. The family, and subsequently, school and peer groups, interpret the prevailing culture and present the child with a social pattern of desirable actions and probable results.
One of the most crucial aspects of the socializing process is “sex typing,” by which a child adopts behavior patterns considered appropriate to his or her sex in a process of copying and encouragement reinforced by reward.
Heredity versus environment
One of the most controversial questions surrounding human development is the extent to which an individual is the product of his or her environment and upbringing on the one hand, or of genetic inheritance on the other. The question of “sex typing” can be seen as a specific example of this. The controversy affects such aspects of development as personality, aptitude, and intelligence particularly. Although various tests have been carried out, for example, on identical twins raised in the same or different environments, findings remain inconclusive. All that can be said with certainty is that heredity, culture, socialization, economic factors, and experience all play important roles in transforming the newborn infant into an adult individual.