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In this volume, which is probably the most complete and most important study ever made of the biology, mentality and incidence of twins, a leading biologist, an educator and a statistician have combined. The book includes three parts devoted to the biology of twins, a comparison of fifty pairs of identical twins and fifty pairs of fraternal twins and, finally, a study of nineteen pairs of identical twins separated in infancy. The work concludes with a summary that indicates the extent to which character is determined by constitution and by environment, the final conclusion of the authors being that what heredity can do environment can also do. Notwithstanding the inability of the authors to state any positive conclusions, the study is of immense interest and highly suggestive for all concerned with similar questions.
Twins: A Study of Heredity and Environment. JAMA. 1938;110(10):764. doi:10.1001/jama.1938.02790100062043