The concept of adulthood can be defined from a biological, legal, psychological, or sociocultural perspective. In our society, an adult is generally perceived as a person who has reached the age of 20 or 21.

The study of adult development is fairly new in scientific and social history, having been preceded by the study of development in childhood, adolescence, and later life. Concern for the status and welfare of children during the industrial revolution of the eighteenth and nineteenth centuries in particular gave rise to scientific studies of the biological, psychological, and social development of children. Scientific interest in old age dates back to the beginning of the twentieth century but did not reach full swing until the second half of the century. Numerous statutes concerned with the welfare of children and older adults have been enacted into law since the 1960s.

The scientific study of human development has its roots in Mendel's pioneering studies of inheritance and Darwin's theory of evolution. The theory of evolution proposed that complex organisms evolve from simple ones by a process of natural selection. Associated with this theory is the principle of epigenesis-the sequential expression of the genetic substrate in physical structure and function. Genetics interacts with experience to shape all human characteristics and behavior. The maturation of motor abilities such as walking is affected by training and practice when the child is optimally ready to learn.

For purposes of tracing the development of human beings from conception to death, it has proved convenient to separate development into a series of stages: prenatal stage, infancy, early childhood, middle childhood, adolescence, early adulthood, middle adulthood, and later adulthood. The transition from one stage to the next is gradual rather than abrupt and does not occur at the same age for all individuals. Certainly, the stages of early and middle childhood are critical to the development of the individual, but development in both the physical and psychological spheres continues on into adulthood.

Research methods employed in the study of human development include observation, interview, surveys, correlational studies, experiments, and quasi-experiments. The two most basic quasi-experimental procedures applied in developmental research are longitudinal and cross-sectional approaches. A longitudinal study reevaluates groups of individuals of the same age at different times, whereas a cross-sectional study evaluates groups of different ages at the same time. Unfortunately, longitudinal studies confound age with time of measurement, and cross-sectional studies confound age with cohort (generational) differences. Schaie's (1977) most efficient design, which involves a combination of cohort-sequential, cross-sequential, and time-sequential analyses, provides a way of assessing the interactions between age, cohort, and time of measurement in their effects on the criterion variable. Whereas longitudinal studies are prospective, information concerning development can also be obtained retrospectively from case histories and other records.

Various statistical procedures are employed in analyzing the results of developmental investigations. Correlation, analysis-of-variance, multiple-regression, and multivariate statistical analyses are among the most widely used statistical methods for this purpose.

Research on adult development and aging is sponsored by numerous public and private agencies and foundations. Prominent among the federal agencies concerned with research on aging are the NIA, the Administration on Aging, and the Office of Nursing Home Affairs. Professional organizations such as the American Geriatrics Society and the Gerontological Society of America serve as forums for scientific studies of aging. The principal legal advocate for the needs of older Americans and the provision of benefits and services to them is the American Association of Retired Persons.

Organizations of professional scientists and practitioners, such as the APA, subscribe to a code of ethics concerning the behavior of their members. Such a code includes statements on prohibitions and sanctions concerned with the care and treatment of research participants. The necessity for considerate treatment of and respect for research participants, obtaining their informed consent, and maintaining confidentiality of the results identified by the names of the participants are all important features of such a code.

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