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.