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The study of age-related changes in behavior from birth to death. Developmental psychologists attempt to determine the causes of such changes. Most research has concentrated on the development of children, but there is increasing interest in the elderly, and to a lesser extent in other age groups. Although most developmental work examines humans, there has been some work on primates and other species that would be considered unethical with human beings. Thus the sensory deprivation of kittens and the separation of monkeys from their mothers have provided information about abnormal perceptual and emotional development, respectively.
Developmental psychologists who study children rely more upon careful observation in natural settings than upon laboratory experiments. Under these circumstances, only partial conclusions can be drawn about the causes of development. The field has been dominated by descriptive research, with increasing attempts to explain developmental phenomena by the use of animal experiments or by statistical methods. In longitudinal research, a group of individuals is studied at regular intervals over a relatively long period of time. This contrasts with cross-sectional research, where individuals of different ages are studied at the same time. Conclusions from the two types of research may differ. Finally, case studies, that is, close and extensive observations of a few subjects, have been relied upon by important developmental theorists such as S. Freud and J. Piaget.
An explanation of developmental changes requires a judgment as to the relative importance of genetically programmed maturation and environmental influences. Although most developmentalists believe that genetic endowment and environmental experience interact to account for behavior, the degree to which either affects a particular behavior is still often debated. This issue has important implications for the success of environmental intervention in the face of genetic constraints. For example, the influence on children of parental speech versus genetic programming in language acquisition is much debated, as is the origin of gender differences in behavior. See Behavior genetics
Developmental psychology is divided roughly between those who study personal–social (emotional) development and those who study intellectual and linguistic development, although there is a small but growing interest in the overlap between these two aspects of personality, known as social cognition. The study of personal-social development in childhood is dominated by the theory of attachment formulated by J. Bowlby and extended by M. Ainsworth. In adolescence and adulthood, E. Erikson's theory of psychosocial development is prominent. The study of intellectual development at all ages is dominated by Piaget's theory of cognitive constructivism.
Ainsworth defines attachment as “an affectional tie that one person forms to another specific person, binding them together in space, and enduring over time … [It] is discriminating and specific.” It is not present at birth, but is developed. In a word, attachment means love. Attachment behaviors such as crying, smiling, physical contact, and vocalizing are the means by which attachment is forged but are not to be equated with the more abstract, underlying construct of attachment. Attachment theory is strongly based on ethological notions. Thus, attachment is seen as serving a biological function, that is, the protection of infants by ensuring their proximity to (attached) adults. The common goal of attached individuals is proximity. Bowlby was influenced by Freud's psychoanalytic theory of development, but argues that there is a primary biological need to become attached to at least one adult, whereas Freud argued that love for a mother was secondary to her satisfaction of an infant's hunger.
For Piaget, intelligence is defined as the ability to adapt to the environment, an ability that depends upon physical and psychological (cognitive) organization. The adaptation process has two complementary components, assimilation and accommodation. Assimilation refers to the tendency to process new information, sometimes with distortion, in terms of existing cognitive structures. Accommodation refers to the opposite process, that is, the modification of existing cognitive structures in response to new information. An individual strives for equilibrium between assimilation and accommodation, with thought being neither unrealistic (excessive assimilation) nor excessively realistic and hence disorganized (excessive accommodation). See Cognition, Intelligence
For Piaget, cognition gradually becomes abstracted from perception over the course of 12 years. Infants begin cognitive exploration by actively perceiving and reflexively manipulating objects, giving the name sensorimotor period to the first phase of intellectual development. Perception is a key form of early cognitive activity, especially with newborns. The newborn infant can see, hear, smell, taste, and feel much better than previously thought, though sensitivity in these areas improves throughout the first year of life. Between the ages of 18 and 24 months, infants become capable of symbolic representation, occasionally solving problems just by thinking about them. The major accomplishment of the sensorimotor period is object permanence, the realization that objects continue to exist even when not observable. During the next 5 years, sometimes termed the preoperational period, children work on concrete operations such as classifying objects into categories, arranging things in serial order, figuring out causes and effects, or understanding a one-to-one correspondence of numbers to objects counted. They also eventually manipulate reality enough to overcome perceptual illusions such as that an amount of water changes when it is poured from a short wide glass into a tall narrow glass. From 7 to 11 years, children further consolidate their concrete mental operations. At about 12 years, many adolescents enter the final stage of intellectual development: formal operations. They become capable of abstract, logical thought. They understand reality as a subset of possible worlds, and are able to form multiple, systematic hypotheses, involving all possible combinations of relevant variables, in order to explain things.
Many quarrel with Piaget's age assessments of children, but most people accept his sequence of stages as useful for classifying children.
L. Kohlberg's work on moral development spans the chasm between intellectual and emotional development. He studied reasoning about hypothetical moral dilemmas, such as whether a person should steal an unaffordable drug in order to save someone's life. He classified such reasoning in six stages. At birth children are considered to be premoral. By the age of 7, most children are in stage 1, chiefly characterized by the belief that people should act in certain ways in order to avoid physical or other punishment. In 2 or 3 years, children reason primarily in terms of doing things for rewards; this is stage 2. Stage 3 involves reasoning focused less on rewards than on maintaining the approval of others. Stage 4 involves reasoning that unquestioningly accepts conventional rules. Actions are judged by a rigid set of regulations, religious, legal, or both. Most individuals do not develop past this point. A few, however, do reach postconventional moral reasoning, stage 5. These individuals think in terms of moral principles. Rarely, a step higher to stage 6 is reached, governed by original abstract moral principles such as articulaton of the golden rule. Kohlberg argued that moral development is progressive, without regression to earlier stages.
Traditionally, child clinical psychology (abnormal development) and the study of normal development were separate. However, effort is being made to integrate them. Abnormal development is informative about normal processes. The serious disorders of childhood include autism, attention-deficit disorder with hyperactivity, and depression. Viewed another way, abnormal children are either overcontrolled (obsessive-compulsive) or undercontrolled (impulsive, aggressive). Developmental psychopathologists, however, are interested not just in disordered development in childhood, but in abnormal individuals over their lifetime. Such studies can shed light on the effectiveness of treatments and on the way in which disorders such as hyperactivity may be displayed differently at different ages.