The study of twins / Peter Mittler.

  • Mittler, Peter J.
Date:
[1971]
Catalogue details

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

Credit: The study of twins / Peter Mittler. Source: Wellcome Collection.

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    79 Twins reared apart results, but the range obtained for each type of relationship is considerable. This range is likely to be due to the wide range of tests, to the age range of the subjects, and to sub¬ stantial differences in experimental design. It can be argued, however, that these studies, taken as a group, and accepting their heterogeneous design and execution, constitute a for¬ midable case for the role of hereditary factors in 'intelli¬ gence'. Few would now dispute such an interpretation, though there remains much room for doubt about the mechanisms and processes involved. Nevertheless, the med¬ ian values approximate remarkably closely to the theor¬ etical values that would be expected on a genetic hypothesis, with correlations increasing regularly with increasing degrees of kinship. Twins reared apart Studies of separated twins could be of fundamental interest, but there are serious practical difficulties not only in finding separated twins but also in ensuring that they are sufficiently representative of non-separated twins and of the general population. The assumption behind such studies is basically simple: since identical twins share the same genes, any differences between them should be due to environmental factors; the effect of such environmental differences can therefore be conveniently studied in twins brought up in different homes. The practical difficulties and problems of interpretation may be illustrated by considering the main studies. The earliest and best known investigation is that of New¬ man and his associates (Newman, Freeman and Holzinger, 1937) who reported in detail on nineteen pairs of separated MZ twins, as well as fifty M Z and fifty DZ pairs brought up together. A large battery of tests was administered, including the Stanford-Binet and Otis tests, together with tests of educational attainment and four tests of personality and temperament (see chapter 6). Detailed biographical and case histories are also included; indeed, these provide a more i: