Twins : a study of heredity and environment / by Horatio H. Newman, Frank N. Freeman, Karl J. Holzinger.

  • Horatio Hackett Newman
Date:
[1937]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Twins : a study of heredity and environment / by Horatio H. Newman, Frank N. Freeman, Karl J. Holzinger. Source: Wellcome Collection.

    35/398 (page 15)
    PREVIOUS STUDIES OF THE PROBLEM 15 The percentage of monozygotic twins is a little high, being 29 per cent, and the number of opposite-sex dizygotic twins a little less than of same- sex twins. However, the proportions are fairly close to those in the un- selected population, indicating that the authors were fairly successful in securing an uninterrupted series. In general, the authors find many more cases in which both twins are affected among the monozygotic than among the dizygotic twins, the pro¬ portions being 68.3 per cent and 14.9 per cent, respectively. However, they find that the nature and course of the malady is often not identical or even closely similar in the case of identical as well as of fraternal twins. In fact, they report that close similarity or identicalness in the expression of the neurosis is the exception rather than the rule. Instances of similar psychoses, quantitative dissimilarity (i.e., dissimilarity of age of onset, particular symptomatology, course, outcome, etc.), qualitative dis¬ similarity (i.e., one twin having a schizophrenic psychosis and the other some neuro¬ psychiatrie condition belonging to a different clinical group, such as mental de¬ ficiency, epilepsy, etc.), and total discordance (i.e., one twin having a schizophrenic psychosis and the other not affected at all), are to be found among both monozygotic and dizygotic twins, but not with the same relative frequencies. The following general concluions are drawn from this study: 1. In the etiology of so-called schizophrenic psychosis hereditary factors seem to play an important part. 2. The hereditary factors, in themselves, are often inadequate; that is to say, they do not suffice to produce a schizophrenic psychosis. 3. The pathogenic effect of the hereditary factors is not highly specific. Other factors often play a part with resulting dissimilarities of manifestation or total dis¬ cordance of findings even in monozj^gotic twins. 4. Hereditary factors are not always present, therefore not essential, in the etiology of so-called schizophrenic psychoses. The most crucial studies of twins are those which use the method of co- twin control. In these studies the two members of a twin pair (usually identical) are kept under relatively identical conditions, with the exception of a specific form of training, which constitutes the experimental variation. This makes it possible to determine how much change in behavior may be brought about by the specified training, or, conversely, how much simi¬ larity in development obtains in spite of the difference in training. The pioneer studies by this method have been made under the direction of Gesell. Three elaborate experiments have been reported dealing with one pair of identical twins: One on climbing and manipulation by Gesell and Thompson,^® one on memory and three types of motor learning by Arnold Gesell and Helen Thompson, Learning and Growth in Identical Infant Twins (Genetic Psychology Monographs, VI, No. 1 [1929]), pp. 5-120.