The scientific study of human twins began in the 1870s when Sir Francis Galton (1822-1911) published a series of articles arguing that heredity (nature) was a stronger factor than environment (nurture) in determining the respective characteristics of twins. He later suggested that identical twins might come from a single egg while non-identical twins might come from two separate eggs, simultaneously fertilized and implanted. This guess, published in Inquiries into Human Faculty and its Development (1883), was later proved correct, but neither Galton nor his contemporaries had any rigorous evidence to support it.
Human twin studies are important for genetic and psychological research because twins provide a natural control for experiments. Because respect for each twin's feelings, privacy, and personhood is easy for even the best-intentioned scientist to compromise, and because twin research readily evokes eugenics, some twin studies will probably always remain controversial, both ethically and scientifically.
Twin studies ramify into genetic, embryological, biochemical, immunological, behavioral, anthropological, psychological, and sociological aspects. Among the questions asked are: What genetic, biological, or environmental factors cause twinning? How do the inherited and acquired traits of twins differ from those of singletons? How do the traits of identical (monozygotic) twins differ from those of non-identical (dizygotic) twins? Why and how does one twin typically dominate the other? What are the effects when monozygotic twins grow up apart? What are the effects of adoption?
American scientists such as Horatio Hackett Newman (b. 1875), Frank Nugent Freeman (b. 1880), and Karl John Holzinger (b. 1892) at the University of Chicago extended Galton's tradition of careful twin research. Their 1937 work, Twins: A Study of Heredity and Environment, is a landmark in the literature of nature versus nurture. In 1940, capitalizing on the widespread frenzy over the Dionne quintuplets (b. 1934), Newman wrote a semi-popular work, Multiple Human Births: Twins, Triplets, Quadruplets, and Quintuplets, speculating on the biological or genetic causes of their seemingly miraculous birth and survival.
Motivated primarily by eugenics, twin studies took a more sinister turn in Germany in the early twentieth century. Johannes Lange (1891-1938) argued that criminal propensities in one twin increased the likelihood of similar sociopathy in the other. His 1929 book, Verbrechen als Schicksal: Studien an kriminellen Zwillingen (Crime as Fate: Studies of Criminal Twins), was admired by the British eugenicist John Burdon Sanderson Haldane (1892-1964) and translated into English in 1930 by Haldane's wife, Charlotte (1894-1969), as Crime and Destiny.
German studies of twins, heredity, and genetics in the 1930s became inseparable from the politics of race. In 1935, a wealthy Bavarian from a prominent family, a member of the Sturmabteilung (SA) received a Ph.D. in anthropology at the University of Munich with a dissertation entitled Rassenmorphologische Untersuchungen des vorderen Unterkieferabschnitts bei vier rassischen Gruppen (Race-Morphological Investigations of Sections of the Anterior Lower Jaw in Four Racial Groups). He then enrolled at the University of Frankfurt Institute for Hereditary Biology and Racial Hygiene to study under Otmar Freiherr von Verschuer (1896-1969). He joined the Nazi party in 1937 and its Schutzstaffel (SS) in 1938. His second dissertation, Sippenuntersuchungen bei Lippen-Kiefer-Gaumenspalte (Genus Investigations on Cleft Lip, Jaw, and Palate) gained him an M.D. in 1938. As an officer in the medical corps of the Waffen SS on the Russian front, he was twice decorated for bravery, seriously wounded, sent home in 1942 to recover, then assigned to Auschwitz as camp physician, where he arrived on 30 May 1943. This Bavarian was Josef Mengele (1911-1979).
Mengele had a morbid and perverse fascination with twins, and saw in the concentration camp an endless supply of subjects for his experiments. Verschuer arranged for full funding of Mengele's research at Auschwitz, provided that Mengele would send the most significant data and specimens to Frankfurt. Mengele arranged to be the camp's principal "selector," deciding who would die and who would live. He ordered all twins lives spared for his purpose of study.
Mengele's twin studies were marked by arbitrariness, cruelty, and lack of scientific rigor. His work was mostly randomm trial and error, without hypotheses. Among his obsessions was trying to change eye color to blue. These attempts would often result in pain, infection, or blindness among the subjects. Convinced that the inmates at Auschwitz were less than human, Mengele kept his subjects naked so that he could measure and observe them more easily. He would inject, bleed, dismember, irradiate, or transfuse his twins, expose them to diseases, or perform unconventional surgical procedures on them without anesthesia. Mengele often killed one or both twin, dissected the bodies, and sent the results to Verschuer. Mengele's experiments involved over 1,500 pairs of twins. Only about 200 of these twins survived.
After World War II, Mengele escaped to South America. In 1959, prompted by Jewish "Nazi hunters," West Germany issued a warrant for Mengele's arrest. The Universities of Munich and Frankfurt revoked his degrees in 1964. A surviving pair of "Mengele Twins," Eva and Miriam Mozes (b. 1935), founded the international support group C.A.N.D.L.E.S. (Children of Auschwitz Nazi Deadly Laboratory Experiments Survivors) in 1985.
Not all twin research is as inhumane, illogical, or useless as Verschuer's and Mengele's. Yet, because of Auschwitz, reasonable and legitimate twin researchers after World War II experienced some difficulty in restoring its domain of inquiry to the level of approval enjoyed in Galton's time. In 1951, Italian geneticist Luigi Gedda (1902-2000) published the first significant postwar work on twins, Studio dei gemelli (A Study of Twins), a large book that was partially translated into English in 1961 as Twins in History and Science. Twin research gained momentum in the 1960s as geneticists revived Newman's interest in discovering the causes of higher-order multiple births.
In 2001, the leading scientific investigator of twins was Thomas Joseph Bouchard, Jr. (b. 1937), professor of psychology at the University of Minnesota and director of the Minnesota Center for Twin and Adoption Research. Beginning in 1990, his team published copiously on the results of the project they conducted from 1979 to 1999, the Minnesota Study of Twins Reared Apart, wherein they periodically administered batteries of psychological, educational, medical, and dental tests to a large population of monozygotic and dizygotic twins and their spouses. Data was collected and maintained by the Minnesota Twins Registry. From 1987 to 2001, the team did a longitudinal study on twin aging, using subjects between 24 and 66 years old at first appointment.
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