That characteristics of anatomic morphology, endocrine physiology, and brain chemistry would not be instrumental in shaping the development of personality is inconceivable. Biological scientists know that the central nervous system cannot be viewed as a simple and faithful follower of what is fed into it from the environment; it not only maintains a rhythmic activity of its own but also plays an active role in regulating sensitivity and controlling the amplitude of what is picked up by peripheral organs. Unlike a machine, which passively responds to external stimulation, the brain has a directing function that determines substantially what, when, and how events will be experienced.
Each individual's nervous system selects, transforms, and registers objective events in accord with its distinctive biological characteristics.
Unusual sensitivities in this delicate orienting system can lead to marked distortions in perception and behavior. Any disturbance that produces a breakdown in the smooth integration of functions, or a failure to retrieve previously stored information, is likely to create chaos and pathology. Normal psychological functioning depends on the integrity of certain key areas of biological structure, and any impairment of this substrate will result in disturbed thought, emotion, and behavior. However, although biogenic dysfunctions or defects may produce the basic break from normality, psychological and social determinants almost invariably shape the form of its expression. Acceptance of the role of biogenic influences, therefore, does not negate the role of social experience and learning (Eysenck, 1967; Meehl, 1962, 1990b; Millon, 1981, 1990; Millon, Blaney, & Davis, 1999; Millon & Davis, 1996).
Although the exact mechanisms by which biological functions undergird personality disorders will remain obscure for some time, the belief that biogenic factors are intimately involved is not new. Scientists have been gathering data for decades, applying a wide variety of research methods across a broad spectrum of biophysical functions. The number of techniques used and the variety of variables studied are legion. These variables often are different avenues for exploring the same basic hypotheses. For example, researchers focusing on biochemical dysfunctions often assume that these dysfunctions result from genetic error. However, the methods they employ and the data they produce are different from those of researchers who approach the role of heredity through research comparing monozygotic with dizygotic twins. This chapter proceeds to subdivide the subject of development into several arbitrary (but traditional) compartments, beginning first with heredity.
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