Until 451, to be an Athenian citizen, one must be a man born of an Athenian father, and have followed the ephebia from 18 to 20 years old, that is to say be able to defend the city. The ephebia is indeed a military and civic formation that allows the city to ensure its defense without having a standing army; it also protected the city from the risks of tyranny. In 451 BC. J.-C., Pericles modifies the law and grants citizenship on the sole condition of having a father and a mother who are citizens.
Slaves and women considered respectively as property and eternal minors, as well as metics (foreigners) were excluded from the political community, as in most Greek cities. However, if a non-barbarian (i.e. Greek) metic performs great deeds for the city, he could exceptionally receive Athenian citizenship in return for his actions, for a fee of course. Such a decision could only be taken following a vote of the Ecclesia bringing together 6,000 citizens. It is therefore easy to imagine the importance and the rarity of these naturalisations.
Citizenship obviously conferred political power, but also judicial protection (citizens could neither be tortured without prosecution nor be sentenced to torture) and an economic advantage:only citizens could own land. This privilege is explained by the history of Athenian democracy; heir to an aristocratic past, the regime considered agriculture to be the only work worthy of a citizen, and valued the life of an annuitant.
In 451, Pericles restricted access to citizenship only to children of Athenian fathers and mothers united in marriage to cope with the explosion in the number of citizens due to unions between Athenians and Metics, or even slaves.