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.

     Based on the genetic knowledge available in the early 1900s, the approach to changing the human gene pool seemed reasonable. After all, Mendel's beautiful demonstration of dominant and recessive inheritance in plants allowed for the prediction of phenotype among theoffspring of parents with known genotypes. Moreover, animal breeders had been applying disassortative mating to successfully improve their livestock for centuries.
     Couldn't these same principles be applied to improve the human population? Eugenics researchers thought so, and they therefore believed that by carefully controlling human matings, conditions such as mental retardation, psychiatric illnesses, and physical disabilities could be eradicated. Eugenics quickly became an issue of public health that was advocated not only by scientists, but also by physicians and lawmakers. All that was needed were data to verify these assumptions. Such data, however, would never emerge.
     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.