Despite its popularity, the eugenics movement was doomed from the start because most of the traits studied by eugenicists had little genetic basis. Among those characteristics targeted for elimination from the human population were such complex and subjectively defined traits as "criminality," epilepsy, bipolar disorder, alcoholism, and "feeblemindedness," a catchall term used to describe varying degrees of mental retardation and learning disabilities. The possibility that environmental factors (such as poor housing, poor nutrition, and inadequate education) might influence the development of these traits was dismissed.
Over the course of 29 years, the ERO collected hundreds of thousands of pedigrees that documented the heritability of the aforementioned undesirable traits. When direct interviews were not possible, family members were categorized in absentia as either affected or unaffected based on hearsay evidence or on records kept by prisons and psychiatric hospitals. In addition, familial data was sometimes collected by untrained individuals or by researchers who projected their own prejudices onto their work, forcing certain traits to seem more heritable than they actually were. Furthermore, some conditions were so subjectively quantified (e.g., "criminality"), with so much ambiguity in their description (e.g., "feeblemindedness"), that almost any member of any family could be categorized in such a way as to make the data fit a Mendelian mold.
Ultimately, families and individuals were deemed either "fit" or "unfit" based on the eugenic belief that complex human traits were controlled by single genes and therefore inherited in a predictable pattern, just like the seed coat color of Mendel's peas. What eugenicists did not know, however, was that Mendel also observed many other traits in his plants that simply did not fit into defined categories and were therefore omitted from his famous study. These researchers were also unaware that most of the traits they were interested in actually result from interactions between genetic and environmental factors, something that confounds predictions of complex disease even today. So, oblivious to the actual nature of complex human traits, the eugenics movement pressed on, providing "scientific" evidence that undesirable human traits were predictably inherited and could be selected against by curbing reproduction in "unfit" individuals.
