The methodology of twin study has been used for fraudulent purposes by many scientists. Two famous examples are the "Burt Affair" and Otmar Freiherr von Verschuer's work at the Kaiser Wilhelm Institute of Anthropology, Human Heredity, and Eugenics.
The paradigm constructed by twin research was distinguished by a marked conceptual reductionism in four respects: First, it presupposed genetic disposition and environment as analytical categories without demarcating them precisely from each other. Second, the paradigm of the twin method did not itemize the two components of heredity and environment into any subordinate components. The urgent interest [in Germany's Second and Third Reichs] was not in the individual genes, their placement on the chromosomes, or the mechanisms of their propagation, not the reciprocal actions they exerted upon each other, and not the complex connections between individual genes and phenotypical characteristics (expressivity, penetrance, specificity) -- at least not initially. Rather, the subject of interest was the genome, and also the environment, conceived of as black boxes. Third the paradigm of twin research proceeded from the assumption that the two components of heredity and environment interacted additively in the development of characteristics, and that consequently it is possible to break down the process of phenogenesis according to magnitudes of influence and determine the respective importance of heredity and environment quantitatively. The complete processes of interaction between hereditary factors and environmental conditions, and the effects of synergy and emergence that result from this interaction, are ignored completely in this approach -- the question was not even posed as to whether it makes sense at all to conceive of heredity and environment as bundles of factors that can be clearly differentiated, and effective in and of themselves. Fourth and finally, the idea that the elements of the phenotype are dependent variables, which ultimately can be traced back over a complex causal chain to two independent variables, the genome and the environment, resulted in an arbitrary definition of dependent and independent variables used in twin research to address the highly complex characteristics of human beings. In so doing it ran the risk of superficially assigning a gene for -- be it for musical talent, sensation of taste, moral instability, criminality, or schizophrenia.[1]
Christoph Mai proposed the thesis that there was a close connection between the boom in twin research and the strengthening of the race hygiene movement in the 1920's. "Leading German human geneticists," according to Mai, "explicitly determined the goals and practical application of their research under the aspect of their eugenic-race hygiene -- i.e., sociopolitical -- usability [...]. [...] [I]n short, Mai characterizes twin research as a pseudoscience.[2]
A veneer of scientific methodology was used, employing Hermann Werner Siemens' "polysymptomatic similarity diagnoses" (1923), wherein multiple anthropometrically-measured phenotype factors were considered proof of genotype similarity. Anthropometric factors measured included hair color and shape, skin color, color of lanugo (fetal hairs), freckles, telangiectasia, cornification in hair follicles, tongue creases, characteristics of the face, shape of the ear, form of the hand and body type, to give the appearance of differentiating between heredity vs environment.[3]