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When Nara Milanich wrote Paternity: The Elusive Quest for the Father—a history of the scientific, legal, and social conceptions of fatherhood in Western civilization—she wasn’t expecting that her publicity tour would be full of interviewers asking her whether she’s done 23andMe. And, truth be told, she’s not that into the question.

Of course people ask. The direct-to-consumer DNA-testing kits are the latest and trendiest technological development relevant to her subject matter. But given the history she’s just written about how sensitive, fraught, and complicated the subject of biological provenance is, Milanich is particularly attuned to how the question of whether someone has done 23andMe could be an intensely invasive one. People are asking whether she’s spent $99 and sent in a swabbed sample of her saliva to find out whether the family she’s always understood to be her family is genetically related to her or not. The next time it happens, Milanich told me as we sat in her office, in the history department of Barnard College, “I’ve decided I’m going to turn it back on them and say, ‘Have you?

Paternity traces the long history of how society got here, to a place where biological paternity is a patently knowable fact that’s apparently fair game to ask a stranger about. As Milanich explains in the book, certainty of biological paternity is a relatively new concept in the long arc of human history. Though it’s been known for centuries that every person has one biological father, the notion that this biological father could be determined by science first broke through in the early 20th century. Milanich’s book illustrates in detail how paternity testing became both a useful tool and a powerful weapon, depending on whose hands the technology landed in. As she pointed out when we spoke, the history of paternity testing may provide valuable clues as to the potentially huge consequences of DNA testing’s swift proliferation as a consumer product.

Maternity, Milanich notes early on in the book, tends to be a much less mysterious relationship than paternity, given the visible facts of pregnancy and birth. Paternity, by contrast, is invisible, and until the 20th century and the advent of the earliest paternity testing, claims to fatherhood (and denials of fatherhood) were only as strong as the implicated parties’ insistence, and circumstantial evidence.

As a result, Milanich explains, for most of modern history, neither the public nor the courts made any distinction between legal and biological fatherhood. When someone had a father, it was because a man had claimed fatherhood status and behaved as a father was expected to by providing care and shelter. The husband of a wife was generally understood, by society and by most countries’ laws, to be the father of her children and thus responsible for providing for them—a rule that proved frustrating to some men when suspicions about their wife’s marital fidelity arose. The expectation for a husband to also necessarily be a father is why the notion of “stepchildren” didn’t really break into mainstream consciousness until the 20th century. It’s also why, Milanich points out, children born to unmarried mothers and absent fathers were for generations simply considered “fatherless,” and many ended up in the custody of the Church or the state when their mothers couldn’t adequately provide for them.

The fascination with knowing paternal identity tends to be a strong and propulsive one. The once-murky nature of paternity provides the central tension of countless great works of fiction, for example. Even contemporary works such as Min Jin Lee’s National Book Award–nominated Pachinko and Elena Ferrante’s bestselling Neapolitan novels, as well as blockbuster musicals and TV shows such as Mamma Mia and Game of Thrones, rely on secret or unknown biological paternity as a plot device. So perhaps it should come as no surprise that scientists all over the developed world worked for decades to find a way to test for a biological relationship between father and offspring.

The first widespread scientific practice to be considered a “paternity test” was blood testing. The blood groups A, B, AB, and O were discovered around the turn of the 20th century, and over the next few decades, scientists applied Mendelian genetics logic to establish that there were limits to the blood types that combinations of parental blood types could produce. This sort of testing couldn’t definitively establish a biological link between two people, but it could rule out a biological link between people whose blood types were incompatible. It took a while, however, for biological paternity to supplant “social” paternity in the popular imagination as the standard by which to measure whether someone was a father. For example, in a famous trial in California in the early 1940s, the silent-film star Charlie Chaplin was accused of fathering a baby with a much younger woman, a charge he denied. A blood test found that it was impossible for Chaplin to have fathered the baby in question, and the results were presented in court. But after witnesses testified that they had seen Chaplin enter the mother’s home at night and leave in the morning, the jury came back with the verdict that Chaplin was in fact the baby’s father.

“The press, and especially the scientific community, went nuts. [They wrote,] California has declared black to be white and the Earth to be flat; how can they be so ignorant?” Milanich told me. “But ultimately, his paternity was determined based not on his biological relationship with the baby but on his social relationship with the mother.”

Within half a century, scientists would begin to unlock the codes of the human genome and further understand the so-called DNA fingerprint. By the 1980s, lab scientists could—given some time and considerable compensation—test two people for a genetic match that would positively confirm biological parenthood, to 99.99 percent certainty.

The sudden ability to positively identify a father for countless numbers of people whose fathers might have remained uncertain forever had significant repercussions. DNA testing promised a way to identify fathers and hold them accountable for supporting the children they could no longer dispute were theirs. This essentially “privatized,” as Milanich puts it in the book, the duty of providing for children, taking the responsibility away from the state or the Church and giving it to biological fathers. For example, Milanich posited when we spoke that it is likely not an accident that the United States’ “child support apparatus” was largely established around the same time that courts and private citizens began to routinely request DNA paternity testing.

The thing to remember about paternity testing, Milanich emphasized, is that it’s available to everyone, for uses both benign and less so. Just as governments and courts have sometimes used paternity testing to ensure that children get the support they need, authorities have also historically used paternity testing to exert control over citizens. Blood testing, for example, was used in Nazi Germany to try to establish whether certain citizens came from Jewish or Aryan parentage; whether a person was sent to a concentration camp or not, in other words, could depend on that person’s ability to prove non-relation to known Jews.

Paternity testing was also used to regulate Chinese immigration to America in the early 20th century, when immigrants were allowed by law to help their immediate families gain entry into the country. Before the implementation of blood testing, immigrants needed only to tell authorities that they had children who wanted to join them in the United States. When the country began testing for biological parentage between immigrants and their putative family members, it aimed to curb the practice of “paper kinship,” or the sale of immigration opportunities to people willing to pose as family members of immigrants already in the United States. But the measures also made it so that immigrants’ adopted children—or other nonbiological kids in their care or guardianship—could be detained for long periods or denied entry.

Now that the knowledge of biological paternity is available cheaply and discreetly through services such as 23andMe, Ancestry, and MyHeritage, Milanich has a few worries about what effects that rampantly available knowledge might have on American society and American citizens’ lives. For one thing, she noted, it’s getting harder to keep biological relationships a secret even if you want to, which can seriously undermine people’s perceived right to privacy, not to mention their peace of mind.

“There’s been a long-standing discussion over the past century about whether ‘knowing who the father is’ is always a socially valuable piece of information,” Milanich said. “Might it be a destabilizing piece of information? Yeah.” (As my colleague Sarah Zhang has reported, it certainly can, as in the case of the fertility doctor Donald Cline, who was found in 2017 to have impregnated many of his unknowing patients with his own sperm.)

The mass commercialization of genetic testing—or as Milanich likes to call it, “Big Paternity”—has made that information more accessible than ever, “and, at least in the U.S., we’ve sidestepped any public conversation about whether people should have access to these things.” For example, Milanich noted that in the age of Big Paternity, the new ease with which people can learn the identities of even “anonymous” sperm donors might deter men from donating. (As I’ve reported before, lawyers who work in assisted reproductive technology and adoption now advise clients that anonymous sperm donation and closed adoption no longer reliably exist.) It’s for reasons like this, she noted, that some countries—such as France—have virtually outlawed direct-to-consumer genetic testing. (“People [in France] do them anyway,” Milanich added. “They just send them to Switzerland and Spain.”)

Milanich also worries about the potential uses of paternity testing as a tool of governmental authority. Already, she noted, U.S. immigration authorities have been using DNA testing at the U.S.-Mexico border to help determine whether migrant families coming into the United States are indeed biological families (as opposed to unrelated individuals posing as families). Testing is also being used to help reunify parents and children who have been detained separately under the Donald Trump administration’s family-separation policy.

Ultimately, adults today are some of the first-ever humans to be able to discreetly and cheaply find out (or verify) their biological parentage without leaving their home. They are also, however, the first generation of adults to be vulnerable to abrupt, destabilizing violations of privacy thanks to the same widely available technology. That most of the advertising and messaging around direct-to-consumer genetic testing focus on the former could prove, down the line, to have been an oversight.

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