The pater familias, also written as paterfamilias (plural patres familias), was the head of a Roman family. The pater familias was the oldest living male in a household. He had complete control of all family members. The term is Latin for "father of the family" or the "owner of the family estate". The form is archaic in Latin, preserving the old genitive ending in -ās (see Latin declension), whereas in classical Latin the normal genitive ending was -ae. The pater familias was always a Roman citizen.
Roman law and tradition (mos maiorum) established the power of the pater familias within the community of his own extended familia. He held legal privilege over the property of the familia, and varying levels of authority over his dependents: these included his wife and children, certain other relatives through blood or adoption, clients, freedmen and slaves. The same mos maiorum moderated his authority and determined his responsibilities to his own familia and to the broader community. He had a duty to father and raise healthy children as future citizens of Rome, to maintain the moral propriety and well-being of his household, to honour his clan and ancestral gods and to dutifully participate—and if possible, serve—in Rome's political, religious and social life. In effect, the pater familias was expected to be a good citizen. In theory at least, he held powers of life and death over every member of his extended familia through ancient right but in practice, the extreme form of this right was seldom exercised. It was eventually limited by law. 
The Roman household was conceived of as an economic and juridical unit or estate: familia originally meant the group of the famuli (the servi or serfs and the slaves of a rural estate) living under the same roof. That meaning later expanded to indicate the familia as the basic Roman social unit, which might include the domus (house or home) but was legally distinct from it: a familia might own one or several homes. All members and properties of a familia were subject to the authority of a pater familias: his legal, social and religious position defined familia as a microcosm of the Roman state. In Roman law, the potestas of the pater familias was official but distinct from that of magistrates.
Only a Roman citizen held the status of pater familias, and there could be only one holder of that office within a household. He was responsible for its well-being, reputation and legal and moral propriety. The entire familia was expected to adhere to the core principles and laws of the Twelve Tables, which the pater familias had a duty to exemplify, enjoin and, if necessary, enforce, so within the familia Republican law and tradition (mos maiorum) allowed him powers of life and death (vitae necisque potestas). He was also obliged to observe the constraints imposed by Roman custom and law on all potestas. His decisions should be obtained through counsel, consultation and consent within the familia, which were decisions by committee (consilium). The family consilia probably involved the most senior members of his own household, especially his wie, and, if necessary, his peers and seniors within his extended clan (gens).
Augustus' ls legislation on the morality of marriage co-opted the traditional potestas of the pater familias. Augustus was not only Rome's princeps but also its father (pater patriae) and, as such, was responsible for the entire Roman familia. Rome's survival required that citizens produce children. That could not be left to individual conscience:the falling birth rate was a marker of degeneracy and self-indulgence, particularly among the elite, who were supposed to set an example. Lex Julia maritandis ordinibus compelled marriage upon men and women within specified age ranges and remarriage on the divorced and bereaved within certain time limits. The Lex Julia de adulteriis coercendis severely penalised adulterous wives and any husbands who tolerated such behaviour. The Lex Papia Poppaea extended and modified the laws in relation to intermarriage between social classes and inheritance. Compliance was rewarded and exceptional public duty brought exemption, but dictatorial compulsion was deeply unpopular and quite impractical. The laws were later softened in theory and practise, but the imperial quaestio perpetua remained. Its public magistrates now legally over-rode the traditional rights of the family concilium and pater familias. The principate shows a clear trend towards the erosion of individual patria potestas and the increasing intrusion of the state into the juridical and executive independence of the familia under its pater.
As priest of familia, gens and genius
The domestic responsibilities of the pater familias included his priestly duties (sacra familiae) to his "household gods" (the lares and penates) and the ancestral gods of his own gens. The latter were represented by the di parentes as ancestral shades of the departed, and by the genius cult. Genius has been interpreted as the essential, heritable spirit (or divine essence, or soul) and generative power that suffused the gens and each of its members. As the singular, lawful head of a family derived from a gens, the pater familias embodied and expressed its genius through his pious fulfillment of ancestral obligations. The pater familias was therefore owed a reciprocal duty of genius cult by his entire familia. He in his turn conferred genius and the duty of sacra familiae to his children—whether by blood or by adoption.
Roman religious law defined the religious rites of familia as sacra privata (funded by the familia rather than the state) and "unofficial" (not a rite of state office or magistracy, though the state pontifices and censor might intervene if the observation of sacra privata was lax or improper). The responsibility for funding and executing sacra privata therefore fell to the head of the household and no other. As well as observance of common rites and festivals (including those marked by domestic rites), each family had its own unique internal religious calendar—marking the formal acceptance of infant children, coming of age, marriages, deaths and burials. In rural estates, the entire familia would gather to offer sacrifice(s) to the gods for the protection and fertility of fields and livestock. All such festivals and offerings were presided over by the pater familias.
The legal potestas of the pater familias over his wife depended on the form of marriage between them. In the Early Republic, a wife was "handed over" to the legal control of her husband in the form of marriage cum manu (Latin cum manus means "with hand"). If the man divorced his wife, he had to give the dowry back to his wife and her family. By the Late Republic, manus marriage had become rare, and a woman legally remained part of her birth family.
Women emancipated from the potestas of a pater familias were independent by law (sui iuris) but had a male guardian appointed to them. A woman sui iuris had the right to take legal action on her own behalf but not to administer legal matters for others.
The laws of the Twelve Tables required the pater familias to ensure that "obviously deformed" infants were put to death. The survival of congenitally disabled adults, conspicuously evidenced among the elite by the partially-lame Emperor Claudius, demonstrates that personal choice was exercised in the matter.
The pater familias had the power to sell his children into slavery; Roman law provided, however, that if a child had been sold as a slave three times, he was no longer subject to patria potestas. The pater familias had the power to approve or reject marriages of his sons and daughters; however, an edict of Emperor Augustus provided that the pater familias could not withhold that permission lightly.
The filii familias (children of the family) could include the biological and adopted children of the pater familias and his siblings.
Because of their extended rights (their longa manus, literally "long hand"), the patres familias also had a series of extra duties: duties towards the filii and the slaves, but some of the duties were recognized not by the original ius civile but only by the ius gentium, specially directed to foreigners, or by the ius honorarium, the law of the Magistratus, especially the Praetor, which would emerge only in a latter period of Roman law).
Adult filii remained under the authority of their pater and could not themselves acquire the rights of a pater familias while he lived. Legally, any property acquired by individual family members (sons, daughters or slaves) was acquired for the family estate: the pater familias held sole rights to its disposal and sole responsibility for the consequences, including personal forfeiture of rights and property through debt. Those who lived in their own households at the time of the death of the pater succeeded to the status of pater familias over their respective households (pater familias sui iuris) even if they were only in their teens. Children "emancipated" by a pater familias were effectively disinherited. If a paterfamilias died intestate, his children were entitled to an equal share of his estate. If a will was left, children could contest the estate.
Over time, the absolute authority of the pater familias weakened, and rights that theoretically existed were no longer enforced or insisted upon. The power over life and death was abolished, the right of punishment was moderated and the sale of children was restricted to cases of extreme necessity. Under Emperor Hadrian, a father who killed his son was stripped of both his citizenship and all its attendant rights, had his property confiscated and was permanently exiled.
- Familias is an archaic genitive form that survived into classical Latin in this fixed expression.
- Severy, 9–10.
- Frier et al., 18–20, for familia case-law definitions (Ulpian) and relations during and before the Imperial period. Limited preview available via Google Books 
- Parkin & Pomeroy, 72–80. Limited preview available via Google Books  (accessed 24 September 2009)
- Galinsky, 130–2. Augustus couched the changes and similar ones as a restoration of traditional values. In one debate, he reiterated a "misogynistic" address of 131 BCE by the censor Metellus Macedonicus on marriage as necessary to Rome's survival. Limited preview via Google Books: 
- Such as the Julli (Julians) of Julius Caesar. See Beard et al., vol 1, 67–8.
- Severy, 9–10.
- Beard et al., vol. 1, 49: citing Cato the Elder, On Agriculture, in Beard et al., vol. 2, 141, source 6.3a.)
- Bingham, Jane: The Usborne Internet Linked Encyclopedia of the Roman World, p. 45. Usborne Publishing, 2002.
- Frier et al., pp. 88–90.
- Pauline Schmitt Pantel, (ed.) A History of Women in the West, Volume I, From Ancient Goddesses to Christian Saints, p. 133.
- Frier et al., 199.
- Beard, M., Price, S., North, J., Religions of Rome: Volume 1, a History, illustrated, Cambridge University Press, 1998. ISBN 0-521-31682-0
- Beard, M., Price, S., North, J., Religions of Rome: Volume 2, a sourcebook, illustrated, Cambridge University Press, 1998. ISBN 0-521-45646-0
- Frier, Bruce W., McGinn, Thomas A.J., and Lidov, Joel, A Casebook on Roman Family Law, Oxford University Press (American Philological Association), 2004. ISBN 978-0-19-516186-1
- Parkin, Tim, & Pomeroy, Arthur, Roman Social History, a Sourcebook, Routledge, 2007. ISBN 978-0-415-42675-6
- Severy, Beth, Augustus and the family at the birth of the Roman Empire, Routledge, 2003. ISBN 0-415-30959-X
- George Long, "Patria Potestas", in William Smith, A Dictionary of Greek and Roman Antiquities London, John Murray, 1875, pp. 873‑875.
- "Roman Law", in Catholic Encyclopedia New York, Robert Appleton, 1913.
- Olga Tellegen-Couper, "A Short History of Roman Law".