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A review of historical developments in the field of human genetics reveals much conflict between those espousing and hereditarian position and those maintaining an environmentalist point of view. A confusion of political, propaganda, and ideological objectives with those of science has had deleterious effects on the field of human genetics, and upon the willingness of some disciplines to accept genetic contributions to the understanding of human behavior. Modern thinking in behavioral genetics rejects the Nature-Nurture dichotomy as incorrect and meaningless. The phenotype cannot be inherited; it develops as a function of interactions between the genotype and the environment. It is convenient to think of hereditary contributions to behavioral traits as fixing a reaction range; the genotype may be viewed as determining an indefinite but nevertheless circumscribed assortment of phenotypes. There have been numerous studies seeking to ascertain the genetic factors in a variety of mental disorders. The mental disorder most thoroughly studied from a genetic standpoint is schizophrenia. Three recent studies of adoptees, two in Denmark and one in America, have indicated the importance of hereditary factors in schizophrenia. Biological relatives of adoptees diagnosed as schizophrenic were more likely themselves to have suffered schizophrenic-spectrum disorders than were biological relatives of non-schizophrenic adoptees. Adoptees whose biological parents were schizophrenic or manic depressive were more likely to themselves be diagnosed schizophrenic than were adoptees whose biological parents had no psychiatric history. Children born to hospitalized schizophrenic women and who were placed for adoption shortly after birth were more likely to be schizophrenic than were matched control adoptees born to non-schizophrenic mothers. In studies conducted in Denmark, Schulsinger has also employed the adoptee method in studies of psychopathy. Diagnostic criteria were developed to reliably identify 57 cases of psychopathy from among 507 adoptees with known mental disorders. A non-psychopathic group of matched control adoptees was also formed. Mental illness among the relatives of these adoptee cases (biological and adoptive relatives) was ascertained through a careful search of psychiatric records. Psychopathic spectrum disorders were found to be more than twice as prevalent among biological relatives of the psychopathic index cases than was the case for biological relatives of the control cases. The percentage of affected cases among adoptive relatives was nearly equal for both study groups. Thus, this study provides a demonstration of the possible importance of genetic factors in the etiology of psychopathy. Other attempts to assess hereditary and constitutional aspects of criminal behavior have been made. These include twin studies and also Eysenck's study of the heritability of behavioral traits including extraversion, introversion, and emotionality.