For a Jewish genetics researcher, being told in print that “Hitler would certainly have been very pleased” by your work can’t be pleasant. But that’s what happened in 2010 to Harry Ostrer, a geneticist at the Albert Einstein College of Medicine, when he and his colleagues published a study showing that Jews in three different geographical areas had certain collections of genes that made them more biologically similar to one another than they were to non-Jews in the same regions. The work also showed that Jews around the world could trace their ancestry to a group of people who lived in the Middle East 2,000 years ago; that meant, however, that certain genetic signatures could be used to identify Jews, indicating that Jews share a common biological identity beyond their religious affiliation—which is what inspired the Hitler crack.Jews, the work of Ostrer’s group and another team found, are as closely related genetically as would be expected for typical fourth or fifth cousins. “I would hope that these observations would put the idea that Jewishness is just a cultural construct to rest,” Ostrer told Science magazine at the time.Ostrer’s new book, Legacy: A Genetic History of the Jewish People, goes further, making a convincing case that there is, in fact, a biological basis for Jewishness: “[Jews] can be said to be a people with a shared genetic legacy,” he writes, “although not all Jews share the same genes, nor is having part of that legacy a requirement for being Jewish.” Although Ostrer gathers the evidence succinctly, the book is unlikely to sway his most ardent critics, scientists such as Tel Aviv University’s Shlomo Sand, the historian who invoked Hitler’s memory to describe Ostrer’s research. But that may not be Ostrer’s main goal. For non-scientists who are curious about their genetic heritage, the book will open up rich avenues of exploration, along with a history lesson about the development of modern genetics—as well as provocative discussion about how the choices Jews are making today about their mates and their children will affect the future of Jewish genetics: Even as Ostrer argues for Jews’ common biological history, he sees the future of Jewish genetics going in a very different direction.Over the past century, the sometimes-arcane debate within the scientific community over whether Jewishness is biological or cultural has been almost Talmudic, with various groups interpreting the data in different ways. Jews have higher IQs than others, some research has found, suggesting a genetic link among Jews; or maybe, other research has countered, that’s only because Jews have learned to take IQ tests better. Jews have a higher incidence of mental illness, or maybe only certain kinds of mental illness, depending on the study. In many cases, when researchers thought they had found some real biological trait more common among Jews, they realized later that they had “keys under the streetlight syndrome”—“ascertainment bias,” in scientist speak: If you’re only looking under the streetlight for your keys, that seems to be the only place you ever find them. In other words, when scientists start their research by looking for more evidence to confirm something they already think is true, they can be less likely to take other explanations into account, making their conclusions misleading.The ability to sequence genomes quickly and more and more cheaply has accelerated the field in recent years, leading to new theories. Sand, for example, argues that today’s Jews are all descended from Khazars, an idea Ostrer finds unsupported by the evidence, as he explains in the book. And for some researchers, the question of “who is a genetic Jew” is less important than “who has Jewish genetic diseases,” since a major part of what Jews think about in terms of their genes involves Canavan, Gaucher, and Tay-Sachs diseases, and risk factors such as BRCA, a genetic mutation linked to breast cancer. (It’s not just Jews who are wondering what’s in their genes, of course, as books such as Robert Klitzman’s Am I My Genes? make clear.)In Legacy, Ostrer traces a century of physical anthropology and genetic research, with all of its twists and turns, and sometimes heated political battles. Ostrer’s book is very much focused on the science and scientists. He explains that he abandoned his plans to write about patients and genealogists in favor of telling the stories of “scientists and physicians who made the discoveries.” The approach allows him to follow the science more closely, although it sacrifices some of the narratives that may connect with readers. Ostrer clearly knows his subject inside and out, and it shows, but readers may find themselves wishing he had replaced some of the details about experiments and genetic theories with stories of some of the people with these genetic traits.Ostrer takes plenty of opportunities to tie history and culture to science, however. English majors may remember Daniel Deronda, George Eliot’s last completed novel, whose protagonist becomes Jewish after rescuing a Jewish singer from suicide. Reading Eliot’s book was a life-changing event for one of the main subjects in Ostrer’s book, physical anthropologist Joseph Jacobs. In the 1880s, Jacobs, who became editor of The Jewish Encyclopedia, used knowledge gleaned from an apprenticeship with Charles Darwin’s cousin, Francis Galton, to study physical characteristics of Jews and conclude that they were, in fact, a race. Based on Jews’ physical characteristics, Jacobs wrote in The Jewish Encyclopedia: “The remarkable unity of resemblance among Jews, even in different climes, seems to imply a common descent.” And yet Jacobs’ claims were opposed to those of Maurice Fishberg, his contemporary. Somehow, despite saying that “one can pick out a Jew from among a thousand non-Jews without difficulty,” Fishberg rejected the concept of a Jewish race. There had been too much intermingling with gentiles, he said.Neither Jacobs nor Fishberg, however, had the genetic tools necessary to dive deep into the molecular underpinnings of genealogy. (In fact, they both died decades before the discovery of DNA.) In the last two decades, scientists have used the male-only Y chromosome to speculate about the origins of Cohanim, at one point excitedly announcing that they had found evidence that the founder of this genetic line had lived during Temple days. But Ostrer notes that this enthusiasm “has been tempered by other observations,” just as the work of other scientists has been woven into a tapestry that none of them could have imagined in their time.Continue reading: The future of Jewish geneticsOstrer’s careful and understated analysis of the evidence makes his arguments convincing. He is nuanced and doesn’t pretend that science has all of the answers about how Jews should feel about their identity. “The stakes in genetical analysis are high,” he writes, noting that they touch “on the heart of Zionist claims for a Jewish homeland in Israel.” Admixture with non-Semitic groups, in fact, “may absolve Jews from Christ-killing,” he writes. And he rejects the idea that his and others’ work would have pleased Hitler: “We were not seeking to develop a hierarchy of human groups nor attempting to eliminate individuals on the basis of their having ‘undesirable’ genes or traits, as the Nazis had.”So, if Jews have common genetic markers, can a gene test (like the ones already being marketed) really tell you if you’re Jewish? “[T]here is no rigorous genetic test for Jewishness, nor would the geneticists who have conducted studies in recent generations propose that one should be created,” Ostrer writes. “Moreover, such a test would not replace the religious definition of who is a Jew.” The Israeli Law of Return, for example, doesn’t have a genetic requirement.And even if such a test existed, it would need constant updating. Because even though Jews have maintained certain common genetic traits for millennia, those traits are likely to change at a rapid pace in the coming years. Jews are intermarrying at increasing rates. One of the genetic studies Ostrer quotes found that about 30 to 50 percent of couples that include Jews are now interfaith, compared to one in 200 couples per generation, on average, over much of human history. And people with a predilection for diseases that might have killed them before they passed on their genes are living longer, thus keeping certain traits in the Jewish gene pool at higher rates than would be expected from natural selection over time. Today’s Jews “are making spousal choices and using genetic information for disease treatment, disease prevention, and embryo selection to determine who future Jews will be,” Ostrer writes.Jewish genetics, in other words, will change over time just as Judaism’s spirituality, social values, and culture have changed over the centuries. All of those characteristics help answer the question of what makes someone Jewish, but what made someone Jewish a thousand or even a hundred years ago is likely not how someone asking that question in a hundred years will answer it. Ostrer quotes Albert Einstein, who said in 1955 that his “relationship to the Jewish people has become my strongest human tie.” He also wrote: “In the philosophical sense there is, in my opinion, no specific Jewish outlook.”***Like this article? Sign up for our Daily Digest to get Tablet Magazine’s new content in your inbox each morning.Ivan Oransky, a physician, is executive editor of Reuters Health. He teaches medical journalism at New York University, and blogs at Retraction Watch and Embargo Watch.