The methodology of twin study

Twin study

Twin studies help disentangle the relative importance of environmental and genetic influences on individual traits and behaviors. Twin research is considered a key tool in behavioral genetics and related fields...

 has been used for fraudulent purposes by many scientists. Two famous examples are the "Burt Affair" and Otmar Freiherr von Verschuer

Otmar Freiherr von Verschuer

Otmar Freiherr von Verschuer was a German human biologist and eugenicist concerned primarily with "racial hygiene" and twin research...

's work at the Kaiser Wilhelm Institute of Anthropology, Human Heredity, and Eugenics

Kaiser Wilhelm Institute of Anthropology, Human Heredity, and Eugenics

The Kaiser Wilhelm Institute of Anthropology, Human Heredity, and Eugenics was founded in 1927. The Rockefeller Foundation supported both the Kaiser Wilhelm Institute of Psychiatry and 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.

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.

A veneer of scientific methodology was used, employing Hermann Werner Siemens

Hermann Werner Siemens

Hermann Werner Siemens was a German dermatologist who first described many skin diseases and was one of the inventors of the twin study.-Biography:...

' "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.