Adoptio was legally the simplest of the three, the passing of someone in patria potestas from the power of one person to that of another.** Adrogatio meanwhile, was far more complicated. This involved the adoption, and therefore return to being under patria potestas, of a man*** who was already sui iuris and therefore legally under no one’s power. But the caput (status/rights) of a Roman citizen could not be negatively affected except with the involvement of a court. In other words, adoption by adrogatio couldn’t happen without both the potential adopter and the adoptee presenting a satisfactory case and having it voted on, with the possibility of conditions being imposed. Originally this could only happen in the city of Rome. As time went on, restrictions were loosened; people didn’t have to go to Rome and one only had to go before the pontifices. Some may have found their way around the restrictions entirely.
What is now called “testamentary adoption” was its own special case. A person could name someone else in their will as their heir on the condition that the inheritor took the dead person’s name. This condition though, could be avoided, especially when the person making the will was an enemy of the state. Whether or not it made the new heir part of a different family was unclear and could be legally challenged. This was the only form of adoption open to women as adopters and was a way a woman could pass on her name and bring someone into her family.
*The wife of an adoptive father was not legally the new son’s mother.
**I’ll get more into the technicalities of this when I talk about adoption as a familial and political strategy.
***Women couldn’t be adopted by adrogatio.
Corbier, Mireille. "Divorce and Adoption as Roman Familial Strategies." In Marriage, Divorce, and Children in Ancient Rome, edited by Beryl Rawson, 47-78. Oxford: Oxford University Press, 1991.
Adoptio - Lacus Curtius
Lindsay, Hugh. Adoption in the Roman World. Cambridge: Cambridge University Press, 2009.
Frier, Bruce. Review of Lindsay, Adoption in the Roman World - Bryn Mawr