“No matter how old you are,” Johnson said, “you never lose the desire for a family.”

Adoption can help provide a feeling of stability, says Deb Castaldo, an adoption therapist in Englewood, N.J., and a faculty member at the Rutgers University School of Social Work.

“Why would someone want to do this as an adult? Many reasons. First, wanting to remove the stigma of not having a family, of not having a feeling of permanence,” she said. “You can imagine what it’s like for someone who has no sense of belonging.”

In addition to grown foster children yearning to legalize an emotional link long felt with their foster parents — or grown stepchildren with their stepparents — same-sex couples who can’t legally marry are also using adult adoptions to ensure that when one partner dies, his or her split of family money goes to the other partner.