Adoptio is a legal act by which a Roman citizen enters another family and comes under the patria potestas of its chief. Since only a paterfamilias (see patria potestas) could adopt, women could not (except in later law by imperial grant). When the adopted person, male or female, was previously in the paternal power of another, the act was adoptio; when a male who was not in paternal power but himself the head of a family, it was adrogatio. Women could not be adrogated. Both acts involved a deminutio capitis minima, a reduction of legal status.
Adrogatio fused two families, for with the adoptee (adrogatus) all under his power (potestas, manus) and his property pass into the family of the adopter (adrogator). In early times adrogatio was publicly validated by a vote of the curiate assembly, preceded (since it extinguished a family and its cult) by an investigation by the pontiffs; by the time of Cicero, 30 lictors represented the curiae (see curia (1)). Since the assembly met only in Rome, adrogatio could take place only there, until (by Diocletian's time at latest) imperial rescript replaced the vote.
Adoptio of a person in power (of any age) was a more private act performed before a praetor or governor. Its form was the same as that of emancipation (emancipatio, the release of a child from power), except that after the third sale the buyer did not free the child but collusively claimed that he/she was in his power. Under Justinian, these formalities were replaced by a declaration before a magistrate.
The effect of both adoptio and adrogatio was to place the adopted person for all legal purposes in the same position as if he/she had been a natural child in the power of the adopter. The adoptee took the adopter's name and rank and acquired rights of succession in his new family, losing those held in the old family. ‘Adoption imitates nature’ (Inst. Iust. 1. 11. 4), so an adoptive relationship barred marriage and the adopter had to be older than the adoptee (a rule allegedly flouted by the adoption of P. Clodius Pulcher). Bachelors and men physically incapable of reproduction (except if they had been castrated) could adopt. Adoption could be reversed by emancipation.
Adoption (of both kinds), since it created paternal power and continued the agnatic family, was originally the prerogative of men only. Adoption by women, ‘to console them for the loss of children’, was allowed by later emperors, as was adrogation of women: this shows a new conception. Safeguards grew up, especially for young children and their property.
Since adoption destroyed the adoptee's succession rights in the old family and a subsequent emancipation would destroy rights in the new family, Justinian drastically changed adoptio to allow the adoptee to retain rights in the old family, except where the adopter was a close relative, e.g. maternal grandfather.
The testamentary adoptions recorded in non-legal sources in the late republic and Principate apparently created only an obligation (from which the praetor could give dispensation) to take the testator's name. So T. Pomponius Atticus inherited from his mother's brother Caecilius and became formally Q. Caecilius Q. f. Pomponianus Atticus (Cic. Att. 3. 20); the future emperor Ser. Sulpicius Galba for some time bore a name taken from the family of his stepmother, Livia Ocellina: L. Livius Ocella (Suet. Galba. 3–4). Caesar's adoption of his great-nephew C. Octavius (see augustus) was ratified by a posthumous adrogatio.
Adoption of adult men was a convenient recourse for childless aristocrats and for emperors in need of successors.