McKinney, Sanchez, and tens of thousands of others are a distinct group: Younger than the many who never knew their donors (or never knew they had one), but older than those whose donors understand they might someday hear from their offspring. The revelations and relationships that result from the new knowledge they’re gaining as adults—of donors, of half-siblings—can change who they believe themselves to be and, in some sense, who they are.

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For McKinney, wondering about her origin story began when she was a teenager, after she learned about her parentage. “I remember crying when I found out,” McKinney says. “I was not happy about it. I remember saying, ‘I feel like a science experiment. I wasn’t even made from love between two people.’” Soon, though, she began to speculate about the other half of her genetic line. What did her donor look like? Was his personality like hers?

There’s a name for that feeling—that curiosity, that sense of a missing piece, that anxiety that some dormant aspect of themselves might one day show up and have no traceable root. In 1964, the psychologists Erich Wellisch and H.J. Sants, who studied and treated troubled adoptees, understood the lack of knowledge of one’s genetic background to induce a state of what they called “genealogical bewilderment.” Wellisch and Sants argued that not knowing one’s ancestry could stand in the way of developing a clear mental image of one’s body, which they argued was necessary to developing a sense of identity. They also believed genealogical bewilderment could stunt the development of feelings of belonging.

Adoptees are, of course, different from donor-conceived children; for starters, adoptees often have to cope with feelings of unwantedness by both biological parents, while donor-conceived kids know their very existence comes from at least one parent’s deep desire for a child. But the thinking then was (and, to an extent, still is now) that both arrangements leave kids wondering about their parentage in a similar way.

Sperm donation—a man donating his sperm to conceive a child who for all intents and purposes belongs to parents other than him—has been happening for centuries. One of the earliest recorded cases of sperm donation in the Western world was in 1884, when a doctor in Philadelphia inseminated a woman with sperm donated by his “most attractive” medical student. The practice became more popular during the Baby Boom years after World War II, though it often remained a secret parents kept from their children and from the world—“both to protect the man from the stigma of infertility and to protect the child from the stigma of illegitimacy,” according to a 2008 article in the quarterly The New Atlantis. For the most part, secrecy was the norm until around the turn of the millennium.