On his 62nd birthday, the emperor Hadrian - tormented by an illness that had crippled him and left him subject to bouts of murderous rage - convened a council of advisers at his bedside. Painfully shifting his dropsy-stiffened limbs, he delivered a short address about his succession plans. The historian Cassius Dio (who wrote nearly a century later) gives this version:

"I, my friends, have not been permitted by nature to have a son, but you have made it possible by legal enactment. Now there is this difference between the two methods — that a begotten son turns out to be whatever sort of person Heaven pleases, whereas one that is adopted a man takes to himself as the result of a deliberate selection. Thus by the process of nature a maimed and witless child is often given to a parent, but by process of selection one of sound body and sound mind is certain to be chosen. For this reason I formerly selected Lucius before all others...But since Heaven has bereft us of them, I have found as emperor for you in his place the man whom I now give you, one who is noble, mild, tractable, prudent, neither young enough to do anything reckless nor old enough to neglect aught, one who has been brought up according to the laws and one who has exercised authority in accordance with our traditions, so that he is not ignorant of any matters pertaining to the imperial office, but could handle them all effectively..." (69.20)

Thus Hadrian announced his plans to adopt the man we know as Antoninus Pius, who would succeed him a few months later.

Roman adoption might best be conceptualized as an oligarchic society's response to demographic realities. Thanks largely to cripplingly high rates of child mortality, many Roman families failed to produce a male heir. Adoption allowed a childless man to perpetuate his family- a concern to all classes, but most pressing and visible among the aristocracy. There were two legal forms of adoption - one for those who had previously paterfamilias of their own family, another for those who had been under the paternal power of another - but the upshot in each case was to make one man the legal son of another.

Legal adoption seems to have developed fairly early - probably by the fourth century BCE. It was certainly well-established by the middle Republic, when the general and statesman Aemilius Paulus negotiated the adoption of two of his sons into prominent families:

"So then Aemilius, having divorced Papiria, took another wife; and when she had borne him two sons he kept these at home, but the sons of his former wife he introduced into the greatest houses and the most illustrious families, the elder into that of Fabius Maximus, who was five times consul, while the younger was adopted by the son of Scipio Africanus, his cousin, who gave him the name of Scipio." (Plut., Aem. 5.5)

The institution of adoption was famously abused / creatively applied by Cicero's nemesis Clodius, who had himself adopted into a plebeian family to make him eligible for the tribunate. We also hear more and more about testamentary adoptions (posthumous adoptions by will) in the late Republic, which (unlike the two legal forms of adoption) entailed only taking the name of one's new "father," not formally joining his family. The most famous instance of this is of course the future Augustus' adoption by his great-uncle Julius Caesar. Adoption continued to evolve in the imperial era, when women were allowed to adopt for the first time. It also became, as we have seen, a principle of imperial succession - though no emperor ever passed over his natural son in favor of an adopted one.