“The Jukes” found favor among penologists, social workers and social gospellers, who combined a humanitarian commitment to the poor with a scientific approach to charity. One admirer was the Rev. Oscar Carleton McCulloch of the Plymouth Congregational Church in Indianapolis. Inspired by Mr. Dugdale, Mr. McCulloch sought to make his own philanthropy scientific through research on Indiana’s poor, some of whom came through his church doors.

One result was “The Tribe of Ishmael: A Study in Social Degradation,” which appeared in 1888. In it, Mr. McCulloch answered a gigantic question in the burgeoning field of heredity at a time when the concept of “race” divided not just white and black, but white people themselves: How could Americans of supposedly the finest racial stocks — English, Saxon and Anglo-Saxon — engender a long history of pauperism and crime?

The answer, Mr. McCulloch said, still lay in blood, but in blood of the wrong kind. He wasn’t being original when describing the English ancestors of the Ishmaelites as the “old convict stock which England threw into this country in the 17th century,” but he was among the first to put it into a social-scientific context.

That logic was largely superseded in the 1910s by a seemingly more advanced approach to the persistence of white poverty: I. Q. testing. Such techniques to quantify intelligence aligned perfectly with an interventionist approach to social betterment, thanks to Charles Benedict Davenport, head of the Eugenics Record Office. Mr. Davenport stood at the hard end of a continuum of eugenic thinking, beside so-called negative eugenicists like Madison Grant, who proposed the literal elimination of people he thought inferior.

Though genes were not yet fully understood, Mr. Davenport had something like them in mind when he argued that intelligence was passed on as a single “unit trait.” That unit trait could be quantified, in the words of an enthusiastic tester, as “the value of a man.” (Mr. Murray made a similar argument in “The Bell Curve,” though he has since moved away from I. Q. hereditarianism.)

Mr. Davenport urged Henry H. Goddard, head of research at the Training School for Backward and Feeble-Minded Girls and Boys in Vineland, N.J., to test his charges. Mr. Goddard found the perfect resident, a 22-year-old he called Deborah Kallikak. His results, published in 1912 in the best seller “The Kallikak Family,” invented the name Kallikak by joining the Greek “kalos” (goodness) and “kakos” (badness).

HER forebear, he deduced, had engendered two families: one upstanding, from a legitimate union, and Deborah’s degenerate one, from sex with a feebleminded barmaid. Rating Deborah a “moron,” Mr. Goddard concluded that she would lack moral judgment and blamed her mental handicap on her ancestry. According to Mr. Goddard, Deborah’s degenerate branch counted 36 illegitimate children, 33 sexually immoral persons (mainly prostitutes), 3 epileptics (epilepsy was considered solely hereditary), 82 dead babies, 3 criminals and 8 brothel keepers. How to block the propagation of hereditary social ills? Sterilization.