A time-honored maxim of scientists who study the physical and behavioral changes in people over time is that development, the sequence in which these changes occur, is the result of the inborn, genetically programmed makeup of the individual interacting with the environment. Biologists applied the term epigenesis to the progressive sequence by which the genetic substrate of an organism is expressed in physical structure and function. Epigenesis does not stop at birth, but, coupled with the experiences of the individual in a sociocultural context, continues to affect development throughout the individual's lifetime.
An enduring question concerns the extent to which genetically based structures and functions simply "unfold" without external influence as opposed to being subject to modification by the environment. Environment, of course, is not limited to intrauterine or even physicochemical events but includes all internal and external, physical and experiential stimuli. The assumption that human characteristics and behaviors result from the complex interaction of heredity and environment applies to height, color blindness, intelligence, and, in fact, everything that a person is or does. Precisely how the heredity x environment interaction takes place, and the relative contributions that genes and environment make to it, is the subject of continuing debate. Still, at least with respect to cognitive abilities, personality characteristics, and behavior, it is a foregone conclusion that both sets of factors are essential.
Related to the concept of epigenesis is that of maturation, the process of development that results in orderly changes in behaviors characteristic of a species. Earlier studies of human development seemed to demonstrate that motor skills such as walking are not significantly affected by restriction of movement or special training (Dennis, 1940). However, later research showed that severe restriction of practice and stimulation distinctly retards the development of sitting and locomotion (Dennis, 1960). Developmental psychologists now believe that experience can affect the rate of maturation of all physical abilities, but the magnitude of the effect varies with the individual and the particular ability. Both maturation and training are important in the development of psychomotor skills; special training is more efficient and effective when the readiness of the individual is at an optimum level to profit from it.
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