Eugenic discourses at the turn from the 19th to the 20th century carried strong currency with experimental biologists, medical doctors, and anthropological researchers. Many contemporary scientists worked to understand the conditions of hereditary disease, influences of environmental circumstances, and the biological predispositions of psychiatric illness. Initially, the scholarly approach to twin studies was pioneered by 19th-century British polymath and psychologist Francis Galton (1822-1911), who had been a cousin of evolutionary biologist Charles Darwin (1809-1882). Galton’s contemporary research was published in a substantial article, entitled “The History of Twins” in the Journal of the Anthropological Institute (1875), which already placed the scientific perspective on twin research in the wider context of eugenics assumptions (thereby classifying the relative powers of nature and nurture in human behaviour and physiology).

American biologist Charles Davenport (1866-1944) received the previous writings of Francis Galton in his own eugenics-oriented work in an extensive form. He also showed a particular interest in twin studies in his biological and epidemiological research program at the Eugenics Record Office in Cold Spring Harbor, New York, since it had been created in 1904. In the modern fields of clinical genetics, human genetics, and population genetics, twin studies are now frequently used for scientifically assessing the importance of developmental and behavioural factors along with physiological characteristics influenced by human heredity.

1. Twins and their research uses
The technical details of twin studies are based on an increasing knowledge about human embryology at the turn from the 19th to the 20th century: There are two kinds of twins: two-egg or dizygotic (DZ) twins are formed, when a mother generates two fertile egg cells at the same time and those are then impregnated by two different sperm cells from the father. Two DZ twins, on the one hand, will genetically tend to be as different as two children from the same two parents who are born at another time interval. About half of DZ twins are of opposite sex, and many twin pairs have different colours of their hair and their eyes. One-egg or monozygotic (MZ) twins, on the other hand, are formed when one egg cell is fertilized by a single sperm cell (~ the zygote) which splits into two separate embryos soon after sexual conception. Consequently, two MZ twins have the same genetic makeup and are always of the same biological sex. Often they look very similar by their overall physical appearance, and they have the same hair and eye colour.

In modern psychology and psychiatry, clinical and basic researchers pursue twin studies with a particular interest in obtaining information on the behaviours and mental characteristics of a larger number of twin pairs from the same geographic regions. Researchers, for example, analyze blood samples or DNA tests on each pair of twins to determine if they are monozygotic or dizygotic. In a second step, genetics researchers intend to examine whether the measures of behaviour or test scores appear more similar in MZ than DZ twins. In case that MZ twins present much more similar scores, researchers are likely to relate such findings to similar sets of genes among the investigated twins; thus warranting the assumption that genes would exert a strong influence also on the mind or the behavioural traits of their test persons.

There is nevertheless a critically important supposition that is often made when twin studies are scrutinized and interpreted, viz. that environmental conditions in DZ and MZ twin pairs must be equally similar or dissimilar. Conversely, if the environmental circumstances experienced by genetically identical MZ twins are more similar than those of DZ twins, MZ twins tend to have more similar hereditary attributes and environmental conditions than DZ twins. Such investigations are not seldom practiced from “newgenics” backgrounds. Moreover, it ensues as a critical postulation that greater similarities between the minds or behavioural traits of MZ twins would prove the overall importance of the biological properties of genes regarding the “nature and nurture debate.” Comparable social experiences of twins may instead have contributed to their more similar scores from intelligence and IQ testing.

Many studies have found so far that the experiences of MZ twins are indeed more similar than those of DZ twins. This greater similarity might however even arise from foregoing prenatal conditions, because many MZ twins reside topographically close in the mother’s womb and share the same chorion cyst along with the reception of their nutritional physiology from the same maternal placenta. Postnatally, their comparable appearances mean that they are treated more alike by other people; they are biologically always of the same sex, while boys and girls are usually referred to and regarded differently in Western societies. In recent times, elaborate statistical methods have been developed in an attempt to separate the effects of hereditary attributes and environmental conditions on twins, but the results remain controversial. A better estimate could perhaps be obtained by combining the resulting information with auxiliary data from other relatives and observations from adopted children who shared the same family homes but are not related genetically.

2. Psychiatric eugenics and the origins of twin studies in mental health
Eugenic programs thereby offered a new basis for psychiatric diagnostics, clinical treatment, as well as innovative social and genetic counselling programs for public mental health–––especially during the interwar period (1919-1939). A very prominent protagonist of this research direction was the psychiatrist and eugenicist Auguste Forel (1848-1931) at the University of Zurich in Switzerland. He was a fervent supporter of the moderation movement along with programs against alcoholism, seeking to minimize detrimental environmental influences on the human genetic endowment. Forel approved of the sterilization of the mentally ill and also supported a first biological research centre in psychiatry at the nearby University of Basel under Anton Carl Brugger (1903-1944). In parallel, leading Germany-based psychiatrists–––such as Alois Alzheimer (1864-1915) in Frankfurt am Main, Emil Kraepelin (1856-1926) in Munich, and Nicolai Vladimirovich Timoféeff-Ressovsky (1900-1981)–––explored similar research venues towards biological psychiatry as well as genetic testing.

The ensuing changes in the Central European psychiatric and mental health care system are quite visible in the research program of psychiatric geneticist Franz Joseph Kallmann (1897-1965) from Berlin. He represents an example from biological psychiatry that merged with contemporary twin studies techniques. Kallmann received his MD degree from the latter university in 1919 and from 1928 onwards, he began to work as a neuropathologist in the large psychiatric asylum of Berlin–Herzberge and collaborated with the director of the German Research Institute for Psychiatry, the active protagonist of Nazi euthanasia and eugenics Ernst Ruedin (1874-1952), investigating the inheritance of schizophrenia in twins (~ The Genetics of Schizophrenia, 1938).

3. The transfer of psychiatric twin studies to North America in the 1930s
Kallmann’s emigration to New York, in 1936, was not at all an easy decision for him, particularly since he wholeheartedly approved of the restrictive medical programs in the Nazi public health service, which he viewed as medically advanced. Accordingly, he still held very close ties to his former colleagues and researchers throughout Nazi Germany–––such as Ernst Ruedin himself, along with Karl Bonhoeffer in Berlin, and psychiatric anthropologist Theobald Lang (1898–1957) in Munich. He actively sought for the genetics advice from Ottmar Freiherr von Verschuer (1896–1969) at the Berlin Kaiser Wilhelm Institute for Anthropology, Human Heredity and Eugenics, along with SS physician Josef Mengele (1911–1979) at the Institute for Hereditary Biology and Racial Hygiene at the University of Frankfurt. At the same time, when they began their highly problematic twin studies in psychiatric asylums and later the murderous clinical research in Nazi concentration camps–––such as the KZ Auschwitz in occupied Poland–––, also Kallmann embarked on continued twin studies of schizophrenia and manic–depressive illness on the dataset of 700 siblings in New York State hospitals with the support of the Department of Mental Hygiene.

It was at the New York State Psychiatric Institute (NYSPI), where Kallmann published his hallmark study The Genetics of Schizophrenia (1938), which became the main reference publication for psychiatric twin studies and likewise a foundation of modern psychiatric epidemiology. The material basis of this epoch-making book was based on the earlier data from Berlin–Herzberge, which Kallmann had brought with him when emigrating to Ellis Island. He had showed that siblings of schizophrenics have a 10 times increased risk for being diagnosed with the disorder as well, while the likelihood for other relatives of schizophrenics was only slightly higher than in normal populations. These findings were later confirmed by Scandinavian and American adoption studies (Essen-Moeller, Psychiatrische Untersuchungen, 1941; Slater, A Review of Earlier Evidence, 1968), while Kallmann resumed his central role as an international player in psychiatric research and human genetics. This was emphasized when he became the president (1950/51) of the American Society of Human Genetics.

4. The reception of twin studies in human genetics and mental health
Kallmann’s twin studies were remarkably well received in the emerging field of North American genetics, while the idea of genetic differences in human behaviour did not receive as much support from the psychiatric community itself. In fact, psychiatrists were rather embarrassed by Kallmann’s publications, which openly advocated for eugenic applications of the research findings (~ Eugenic Birth Control in Schizophrenic Families, 1938). What this case shows is nevertheless a double fracture in the historical events: Clearly, the political intentions of Nazism were very different from the US mental health care system in the postwar period, but it is necessary to see the similarities in the scientific approaches in the medical and research landscapes on both sides of the Atlantic. Medical scientists were among the very protagonists who pursued their research aims, who invented racial hygiene and broadened medical genetics, even if the idea of genetic differences in human behaviour found only little support among psychiatrists. These dynamics are reflected in an editorial of the director of the National Institute for Mental Health, Seymour Kety (1915–2000), which states the reluctance to accept the genetic evidence as an integral part of biomedical knowledge in the psychiatric and psychological communities as well, because of “the biophobic and psychophobic attitudes of representatives of the behavioural and biological disciplines, respectively, and the problem of communication that exists between one field and another” (annual reports of the NIMH, 1954). The historical case of Kallmann thus reflects an intricate relation between the community of psychiatric genetics with contemporary eugenics and public health in Europe and North America.

5. Discussion
The eugenics and racial hygiene movements rose to strong prominence in the psychiatric and mental health research communities of the early 20th century. The field of medical genetics was internationally oriented through strong exchanges between leading German, Scandinavian, British, and US researchers. Twin study methodologies had emerged from such contemporary eugenics research contexts, to which racial hygienists from various disciplinary perspectives actively contributed: Ernst Ruedin at the German Research Institute for Psychiatry (Munich), Ottmar Freiherr von Verschuer at the Kaiser Wilhelm Institute for Anthropology, Human Heredity and Eugenics (Berlin), along with his protégé, the SS physician Josef Mengele at the Institute for Hereditary Biology and Racial Hygiene at the University of Frankfurt. The advent of “racial hygiene” and psychiatric eugenics clearly provides an insightful example to understand the interrelationship of internationalization of research, political ideology, along with the dehumanizing aspects of certain biomedical research endeavours and the public health system of the time. Modern histories and discourses about twin studies and the contributions of the genetic perspective to psychiatry and mental health should not spare out but rather critically assess this context of human experimentation in the mental asylums, penitentiaries, and concentration camps within the entrenched forms of eugenics in psychiatry and human genetics research of the time.

Twin studies had definitely been central to the history of eugenics from the very outset. Research findings related to the comparison of the behaviour and the physiology of twins also found their way into the broader social and policy field of “family studies,” which allowed for the developments of opinions about specific physical traits, the inheritance of illness, along with applications for marriage counselling. Twin studies had become increasingly popular in areas of psychology, psychiatry, neurology, internal medicine, and human genetics during much of the 20th century. This was especially stimulated through research on the similarity of behavioural traits, the ways of inheritance of genetic diseases, as well as for the understanding of comparable cellular and microbiological pathways. As such, twin studies are still being actively pursued today. While eugenics assumptions had been quite explicit in the emergence of twin studies at the end of the 19th century, after the middle of the 20th century–––based on the appalling experiences with Nazi eugenics and the widespread international forced sterilization programs–––these views became more and more hidden and implicit. However, similar assumptions about the related and coupled inheritance of character traits as well as diseases are still quite prominent in the perspectives of reproductive medicine and “newgenics” today, when parents are expressing specific wishes about the personal qualities of their offspring or genetic councillors advise on clinical questions of abortion or the delivery of human babies. Twin studies, as we have tried to show in this article, have a fairly sordid history. As a result of such problematic contexts, this history can inform modern-day observers about the very nature of twin studies in the context of reproductive medicine and genetic engineering, to decode the often hidden (new) eugenic assumptions in contemporary research.

-Douglas Wahlsten & Frank W. Stahnisch

• Bryan, Elizabeth, 1983, The Nature and Nurture of Twins. London: Ballière Tindall.

• Galton, Francis, 1883, Inquiries into Human Faculty and its Development. London: Macmillan.

• Kroener, Hans-Peter, 1998, Von der Rassenhygiene zur Humangenetik. Stuttgart: Gustav Fischer.

• Piontelli, Alessandra, 2008, Twins in the World: The Legends They Inspire And The Lives They Lead. New York: Palgrave Macmillan.

• Proctor, Robert, 1988, Racial Hygiene: Medicine under the Nazis. Cambridge, MA: Harvard University Press.

• Ruedin, Ernst, 1916, Studien ueber Vererbung und Entstehung geistiger Stoerungen. Berlin: Springer.

• Stahnisch, Frank W., 2014, “The Early Eugenics Movement and Emerging Professional Psychiatry: Conceptual Transfers and Personal Relationships between Germany and North America,” Canadian Bulletin of Medical History 31 (2014), pp. 17-40.

• Weingart, Peter, Kroll, Juergen, and Bayertz, Kurt, 1988, Rasse, Blut und Gene. Geschichte der Eugenik und Rassenhygiene in Deutschland. Frankfurt am Main: Suhrkamp.

• Wahlsten, Douglas “The Intelligence of Heritability.” Canadian Psychology 35 (1994): 244-258.

• Wright, Lawrence, 1997, Twins And What They Tell Us About Who We Are. New York: John Wiley.