People are discovering the truth about their biological parents with DNA—and learning that incest is far more common than many think.

…

In 1975, around the time of Steve’s birth, a psychiatric textbook put the frequency of incest at one in a million.

But this number is almost certainly a dramatic underestimate. The stigma around openly discussing incest, which often involves child sexual abuse, has long made the subject difficult to study. In the 1980s, feminist scholars argued, based on the testimonies of victims, that incest was far more common than recognized, and in recent years, DNA has offered a new kind of biological proof. Widespread genetic testing is uncovering case after secret case of children born to close biological relatives—providing an unprecedented accounting of incest in modern society.

The geneticist Jim Wilson, at the University of Edinburgh, was shocked by the frequency he found in the U.K. Biobank, an anonymized research database: One in 7,000 people, according to his unpublished analysis, was born to parents who were first-degree relatives—a brother and a sister or a parent and a child. “That’s way, way more than I think many people would ever imagine,” he told me. And this number is just a floor: It reflects only the cases that resulted in pregnancy, that did not end in miscarriage or abortion, and that led to the birth of a child who grew into an adult who volunteered for a research study.

Most of the people affected may never know about their parentage, but these days, many are stumbling into the truth after AncestryDNA and 23andMe tests. Steve’s case was one of the first Moore worked on involving closely related parents. She now knows of well over 1,000 additional cases of people born from incest, the significant majority between first-degree relatives, with the rest between second-degree relatives (half-siblings, uncle-niece, aunt-nephew, grandparent-grandchild). The cases show up in every part of society, every strata of income, she told me.

Non-paywall link