"Ten Years of Research on Twins: Contributions to the Nature-Nurture Problem"

Abstract

I. INTRODUCTIONMany persons have been disappointed by the results of twin studies in which solutions to nature-nurture problems have been sought. There are several reasons. One is the prevalence of studies unequal to the requirements of the problems attacked. Another reason is the tendency of critics to expect too much, either because of lack of appreciation of the complexity of nature-nurture problems or because of failure to realize the limitations of the twin-study technique. This review of methods and results aims not only to indicate some trends of recent contributions, but also to present a picture of the inherent difficulties and complexities of the problems. This review will not be exhaustive in any sense. In so far as is practicable, it will deal critically with methods and results rather than with particular investigations. The material may be summarized under the following topics: classification of twins, abnormalities in twins, studies of twins reared apart, intensive studies of twins reared together, twin resemblances in relation to age, correlational studies of twin resemblances, and environmental influences. Each of these will be discussed in a separate section; in each such discussion a minimum of space will be devoted to the elementary facts about twins known prior to 1928. The main emphasis is upon trends in twin studies, and advances in knowledge since the publication of the Twenty-Seventh Yearbook of this Society. II. CLASSIFICATION OF TWINSThe classification of twins into monozygotic (identical) and dizygotic (fraternal) groups is basic to the whole approach; the last word 1 Clerical assistance in connection with this report was furnished by the personnel of Works Progress Administration Official Project No.

Kurt Gottschaldt's ambiguous relationship with national socialism.

“…There was also a wide range of conclusions drawn from the studies reported. Many (both in America and in Europe) saw evidence that the effects of heredity were substantially greater than those of the environment (Carter, 1940; Woodworth, 1941), whereas others argued for a more balanced view of the relative contributions of heredity and environment (Freeman, 1937; Rexroad, 1932).…”

Studies of identical twins reared apart.

“…When diagnoses are made by practiced experts using all the accepted lines of physical evidence, zygosity can be determined with a high degree of certainty in something like 97 or 98 per cent of twin pairs (2). The less practiced observer, of course, is more likely to make a wrong classification.…”

The growth and prediction of intelligence.

“…For a discussion of biological aspects of twinning, see early chapters in a volume by Newman, Freeman, and Holzinger (57)-2 For studies in this field, see Thorndike(78), Wingfield and Sandiford (95), Hirsch (34), Merriman (55), and Newman, Freeman and Holzinger (57).Carter(9) has reviewed recent studies in this area.…”