In 1912 American psychologist and enthusiastic eugenicist Henry Goddard published his landmark book, The Kallikak Family: A Study in the Heredity of Feeble-Mindedness. This book was the story of two branches of a single family, both descended from the same man. One branch started with his bastard son, was the result of a dalliance with a 'feeble-minded' barmaid; the other branch from his godly wife of good family.
Goddard, to protect the identities of those involved, named them the Kallikaks, from the Greek words καλός (kallos), meaning beautiful, and κακός (kakos), meaning bad. As it happened, the barmaid's children turned out to be the 'bad' ones, and Goddard reported that her descendants were all feebleminded. The 'good' blood produced a family of wealthy lawyers and doctors.
Time has not been kind to the Kallikak study. Part of this is simply due to the advancement of science, as we now know that malnutrition and parental exposure to alcohol affects intelligence, and these might well have been the cause of some of the 'bad' Kallikaks' delays. Goddard's strong opinion that cognitive delays are a matter of poor genetics is questionable, aside from all other concerns.
Even more damaging is Goddard's poor experimental and ethnographic procedure. Goddard worked by tracing the family back from Emma Wolverton (AKA 'Deborah Kallikak'), a ward in Goddard's institution, the New Jersey Home for the Education and Care of Feebleminded Children. Her living relatives were judged to be likewise feeble-minded based on the report of his assistant, who visited their home and could tell that they were "defective stock" just by looking at them (she reported, as supporting evidence, that the children were slack-jawed and their shoes were falling apart). Deceased relatives were researched, apparently, based on second-hand report and intuition.
And finally, it is apparent that Goddard was happy to lie -- or at least, ignore inconvenient truths. Latter researchers found that the Wolverton family tree is well documented, and contains no feeble-minded barmaids; the man Goddard had labeled as a bastard son was actually a second cousin. As it happens, this cousin, and the rest of the 'bad' Kallikaks did fairly well for themselves.
The bastard son, 'Martin Kallikak Jr.' (actually John Wolverton), was literate, as was his entire family, as reported in the 1850 census (96% of New Jersey residents were literate at this time), and was able to expand his farm, buying new plots of land to expand his holdings when he was 33; while this is not as impressive as being a lawyer, it is not the mark of a lazy ignoramus. His family included school teachers, an Army Air Corps pilot, and a bank treasurer; nonetheless, Goddard had no hesitation in declaring that "the whole family was a living demonstration of the futility of trying to make desirable citizens from defective stock through making and enforcing compulsory education laws".
This is a fairly brief overview. Stephen Jay Gould is rather annoyed that Goddard apparently doctored the photos of the Kallikak family to make the 'bad' lineage look more sinister, blacking out the eyes and darkening their mouths. J. David Smith and Michael L. Wehmeyer, in their review Who Was Deborah Kallikak?, point out that the girl who triggered this study was not sent to the home because of intellectual concerns, but because her mother had remarried and her new husband did not care to take on her children. (It is interesting to look at the IQ test used (.pdf) to rate her as low IQ, and consider if it is valid; she was tested at age 21 as having the IQ of a nine year old, but had never had formal schooling aside from in a classroom of "morons"). Emma was also literate, frequently wrote letters to her friends, and was employed in a number of roles within the home, including as a medical assistant and a teacher's assistant in multiple classes. The primary complaint against her seems to be that she was 'wilful'.
Regardless, at the time this book was a national bestseller, and paved the way for generations of scientifically based eugenics and discrimination, from placing children like Emma in institutionalised settings so that they could not act on promiscuous urges (and thus pollute 'racial purity') to decades of compulsory sterilization of the intellectually disabled.