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It has long been known that schizophrenia, manic depressive psychosis, the neuroses, and other deviations of personality and behaviour may be found in much higher frequencies among the relatives of affected individuals than in the population at large. Such concentration within family groups has been attributed to three different types of causation: (a) Similar genetic predisposition. (b) The sharing of a common disturbing environment, (c) Pathological child-rearing practices continued from one generation to the next. In order to examine the genetic hypothesis of causation two main approaches have been adopted, either separately or in conjunction: (1) investigation of twins including at least one affected member, and (2) estimation of frequencies of abnormality in various classes of relatives. A third method, which has not received much attention is the study of dual matings.
The classical twin method is based on a comparison between groups of monozygotic (MZ) and dizygotic (DZ) twins. The former result from the union of a single sperm with one ovum, and have identical genes; the latter are produced from different sperm uniting with two ova and the resultant twins are no more alike genetically than siblings. If the condition is present in both twins the pair is termed concordant, and when the concordance rate is significantly higher in the monozygotic group, the finding is regarded as supporting a hereditary basis for the trait concerned. The underlying assumption is made in concordance
studies that the environment for MZ pairs is not, on the average, more similar than that for members of DZ pairs, and this has been seriously questioned, particularly in psychiatric studies. Critics, drawing on isolated case reports to support their argument, claim that the closer relationship between MZ twins can of itself provide sufficient explanation for their higher concordance rate when considering mental and nervous disorders.
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