Adoption was a possibility for the head of the oikos in ancient Athens, providing he had no legitimate sons. This artificial introduction of a new member into the family was called poiesis or eispoiesis and was analogous to the procedure whereby the city received a stranger into its bosom and recognized him as a citizen. The rights of the adopted person were never exactly the same as those of the natural members of the oikos. He could not, for example, draw up a will, and the blood relatives (anchisteis) of his foster-father took precedence, in the adopted son's inheritance, over his own descendants.

The procedure for recognizing nothoi ex astes could be the same as that for the recognition of adoption: presenting the adopted son to the phratria . Nothoi ex astes were sons born to a mother who was an Attic woman (Atthis), but not the lawful wife. Once recognized, the illegitimate son entered the oikos and was able to take the place of the lawful son. On the other hand, nothoi ex astes could not be recognized, and from the mid-5th century B.C. were not even considered to be citizens.

Paternal power was particularly strong both because of the legislature and because of the social perceptions of the time. Apparently the son's obligation to seek his father's consent before marrying (epibebaiosois) was due to these perceptions more than to its safeguarding in law. Another form of the exercise of paternal power was apokeryxis. The name is enough to show that this process was carried out by a herald, who announced to the whole city the father's decision to exclude his son from the oikos. Such a punishment could of course only be applied after exceptionally severe offences.

Among the rights of a father and kyrios of an oikos were to forbid or compel his wife to abortion, and also to decide to have a child exposed as a foundling, particularly common in the case of baby girls.