Because of the rise of genetic testing sites like 23andMe, Mr. Baden-Lasar now has access to information that his mothers had never considered would be widely available. As this generation of donor children comes of age, they are able find their parent and one another more easily than ever before. It can seem, as Mr. Baden-Lasar puts it in the accompanying essay in the magazine this Sunday, as if they were all assigned at birth to be unwitting participants in “an inadvertent social experiment.” His donor father knows that they exist, but he declined to participate in the project.

As M r. Baden-Lasar worked with the magazine’s editors on the project , there was no set plan other than to photograph as many siblings as possible as quickly as possible. “I was at the whim of other people’s schedules,” he said. “I would jump at any free moment that they had.” For 10 months, he worked largely around the schedules of the more than two dozen siblings, all but one of whom he had never met.

One of his siblings, Mr. Baden-Lasar found out, had roomed with him in a high school program in New York. Some had grown up with siblings of their own; some are only children. One lives in Hawaii; two siblings, who share the donor father and the same mother, live 30 minutes from where Mr. Baden-Lasar grew up in Oakland, Calif. A few siblings are of mixed race. Members of the group came from vastly different socioeconomic backgrounds.

“I was apprehensive about the project in the beginning,” he said, “because news about sperm donation and this world has hit full saturation in the media. It always seems to be reduced to daddy complexes or dramatizing certain things. I wanted the emotional thrust of it to be in the images.”

Mr. Baden-Lasar used a large-format camera for the project — an intentionally retro and challenging device. “So much about the world was already being mediated by online databases and registries and social media; all of this technology,” he said, “and this type of camera just felt completely out of that world.”

This is Mr. Baden-Lasar’s first magazine assignment and his first major photography project. He took a year off from school to work on it and will matriculate at Wesleyan University in the fall as a sophomore.