The role of heredity is usually inferred from evidence based on correlations among traits in members of the same family. Most psychopathologists admit that heredity must play a role in personality disorder development, but they insist that genetic dispositions are modified substantially by the operation of environmental factors. This view states that heredity operates not as a fixed constant but as a disposition that takes different forms depending on the circumstances of an individual's upbringing. Hereditary theorists may take a more inflexible position, referring to a body of data that implicate genetic factors in a wide range of psychopathologies. Although they are likely to agree that variations in these disorders may be produced by environmental conditions, they are equally likely to assert that these are merely superficial influences that cannot prevent the individual from succumbing to his or her hereditary inclination. The overall evidence seems to suggest that genetic factors serve as predispositions to certain traits, but, with few exceptions, similarly affected individuals display important differences in their symptoms and developmental histories (Lives-ley, Jang, & Vernon, 2003). Moreover, genetically disposed disorders can be aided by psychological therapies (Millon, 1999), and similar symptomatologies often arise without such genetic dispositions.
A number of theorists have suggested that the milder pathologies, such as personality disorders, represent undeveloped or minimally expressed defective genes; for example, the schizoid personality may possess a schizophrenic genotype, but in this case
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