Separating the relative contributions of nature (heredity) versus nurture (environment) to human behavior has always been of compelling interest to biologists. American eugenicists, influenced by August Weismann's germ plasm theory, favored the interpretation that behavioral traits are largely determined by genes — which are aloof from environmental influence. French eugenicists, under the lingering influence of Lamark, still favored the notion that environment could modify heredity — and thus behavior.
Although the idea of using twins to approach the issue of nature/nurture in humans first occurred to Francis Galton, he did little work on the problem himself. However, he stimulated others to make use of twin studies, which ultimately provided evidence for the downfall of eugenics.
In the 1920s, Herman Muller evaluated a pair of identical twins who had been raised apart from one another. Although they both had high intelligence test scores, they had been raised in unexceptional middle class families in two different midwestern states. Muller concluded that it would take "an army" of psychologists and geneticists to separate the genetic and environmental components in such twin pairs.
Work in the 1930s by Horatio Newman, Frank Freeman, and Karl Holzinger showed that identical twins raised apart had nearly 100% concordance (agreement) on physical traits — such as appearance, longevity, blood groups — but showed variable concordance on behavioral and personality traits. Notably, twins raised separately had a 15-point difference in IQ — strongly suggesting that intelligence is influenced by upbringing. All this strongly indicated that the eugenicists' simplistic analyses could not hope to account for complex behavioral traits, which are governed by multiple genes and subject to environmental conditioning.
The Nazis took twin studies to a grisly extreme. As director of the Institute of Hereditary Biology and Race Biology at the University of Frankfurt, Omar von Verschuer was famous for his studies of genetic versus environmental influences in twins. His assistant Josef Mengele was posted as senior physician at the Auschwitz concentration camp in 1943. When von Verschuer moved to Berlin to become director of the Kaiser Wilhelm Institute of Anthropology, Human Heredity, and Eugenics in Berlin, he obtained funding from the Reich Research Council to support Mengele's twin research at Auschwitz. One project attempted to identify "specific proteins" involved in a genetically determined response to infection. For this experiment, Jewish and Gypsy twins were injected with identical amounts of typhoid bacteria, then blood samples were taken at regular intervals, and sent to Berlin for analysis. Of the more than 3,000 twins conscripted into twin studies at Auschwitz, only 200 survived the war.