Here's a representative story from the NY Times:
Sperm Donors Can’t Stay Secret Anymore. Here’s What That Means. By Susan Dominus
"To be the biological child of an anonymous sperm donor today is to live in a state of perpetual anticipation. Having never imagined a world in which donors could be tracked down by DNA, in their early years sperm banks did not limit the number of families to whom one donor’s sperm would be sold — means that many of the children conceived have half-siblings in the dozens. There are hundreds of biological half-sibling groups that number more than 20, according to the Donor Sibling Registry, where siblings can find one another, using their donor number. Groups larger than 100, the registry reports, are far from rare.
"Because of the increasing popularity of genetic testing sites like 23andMe, in the past two or three years a whole new category of people, including those who never knew they were conceived via donor insemination, are reaching out to half siblings who may have already connected with others in their extended biological family.
...
"Over time the adoption movement popularized the principle that individuals had a right to know their biological roots, and lesbian couples and single mothers, dominating ever more of the sperm banks’ market, called for greater transparency. In the early 2000s, California Cryobank offered, for a premium fee, an option for parents to choose a donor who agreed not just to be contacted when the offspring turned 18 but to respond in some fashion (though still anonymously if that was his preference).
Sperm Donors Can’t Stay Secret Anymore. Here’s What That Means. By Susan Dominus
"To be the biological child of an anonymous sperm donor today is to live in a state of perpetual anticipation. Having never imagined a world in which donors could be tracked down by DNA, in their early years sperm banks did not limit the number of families to whom one donor’s sperm would be sold — means that many of the children conceived have half-siblings in the dozens. There are hundreds of biological half-sibling groups that number more than 20, according to the Donor Sibling Registry, where siblings can find one another, using their donor number. Groups larger than 100, the registry reports, are far from rare.
"Because of the increasing popularity of genetic testing sites like 23andMe, in the past two or three years a whole new category of people, including those who never knew they were conceived via donor insemination, are reaching out to half siblings who may have already connected with others in their extended biological family.
...
"Over time the adoption movement popularized the principle that individuals had a right to know their biological roots, and lesbian couples and single mothers, dominating ever more of the sperm banks’ market, called for greater transparency. In the early 2000s, California Cryobank offered, for a premium fee, an option for parents to choose a donor who agreed not just to be contacted when the offspring turned 18 but to respond in some fashion (though still anonymously if that was his preference).
By 2010, experts in reproductive technology were starting to note that internet searchability, facial-recognition software and the future of DNA testing would soon render anonymity a promise that the sperm banks could no longer keep. Since 2017, California Cryobank has stopped offering anonymity to its new donors. Donors now must agree to reveal their names to their offspring when they turn 18 and to have some form of communication to be mediated, at first, by the bank."
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And here's an accompanying story, by a man who has now met and photographed many of his half-sibs.