The mos maiorum ("ancestral custom" or "way of the elders," plural mores, with maiorum a genitive plural; cf. English "mores") is the unwritten code from which the ancient Romans derived their social norms. It is the core concept of Roman traditionalism, distinguished from but in dynamic complement to written law. The mos maiorum was collectively the time-honored principles, behavioral models, and social practices that affected private, political, and military life in ancient Rome.
Family and society 
The Roman family was hierarchical, as was Roman society. It is perhaps a matter of perspective as to whether society replicated the power structure of its basic building block, the familia (better translated as "household" than "family"), or whether the male prerogative in a militarized society imposed itself domestically. At any rate, these hierarchies were traditional and self-perpetuating, that is, they supported and were supported by the mos maiorum. The paterfamilias, or head of household, held absolute authority over his familia, which was both an autonomous unit within society and a model for the social order, but he was expected to exercise this power with moderation and to act responsibly on behalf of his family. The risk and pressure of social censure if he failed to live up to expectations was also a form of mos.
The distinctive social relationship of ancient Rome was that between patron (patronus) and client (cliens). Although the obligations of this relationship were mutual, they were also hierarchical. The relationship was not a unit, but a network (clientela), as a patronus might himself be obligated to someone of higher status or greater power, and a cliens might have more than one patron, whose interests might come into conflict. If the familia was the discrete unit underlying society, these interlocking networks countered that autonomy and created the bonds that made a complex society possible. Although one of the major spheres of activity within patron-client relations was the law courts, patronage was not itself a legal contract; the pressures to uphold one's obligations were moral, founded on the quality of fides, "trust" (see Values below), and the mos. Patronage served as a model when conquerors or governors abroad established personal ties as patron to whole communities, ties which then might be perpetuated as a family obligation. In this sense, mos becomes less a matter of unchanging tradition than precedent.
Tradition and evolution 
Roman conservatism finds succinct expression in an edict of the censors from 92 BCE, as preserved by the 2nd-century historian Suetonius: “All new that is done contrary to the usage and the customs of our ancestors, seems not to be right.” But because the mos maiorum was a matter of custom, not written law, the complex norms it embodied evolved over time. The ability to preserve a strongly centralized sense of identity while adapting to changing circumstances permitted the expansionism that took Rome from city-state to world power. The preservation of the mos maiorum depended on consensus and moderation among the ruling elite, whose competition for power and status threatened it.
Democratic politics driven by the charismatic appeal of individuals (populares) to the Roman people (populus) potentially undermined the conservative principle of the mos. Because the higher magistracies and priesthoods were originally the prerogative of the patricians, the efforts of plebeians (the plebs) for access could be cast as a threat to tradition (see Conflict of the Orders). Reform was accomplished through legislation, and written law replaced consensus. When plebeians gained admission to nearly all the highest offices except for a few arcane priesthoods, the interests of plebeian families who ascended to the elite began to align with those of the patricians, creating Rome's nobiles, an elite social status of nebulous definition during the Republic. The plebs and their support of popular politicians continued as a threat to the mos and elite consensus into the late Republic, as evidenced in the rhetoric of Cicero.
The auctoritas maiorum ("ancestral authority") could be evoked to validate social developments in the name of tradition. Following the collapse of the Republic after the death of Julius Caesar, Augustus disguised his radical program under a piety toward the mos maiorum.
During the transition to the Christian Empire, Symmachus argued that Rome's continued prosperity and stability depended on preserving the mos maiorum, while the early Christian poet Prudentius dismissed the blind adherence to tradition as "the superstition of old grandpas" (superstitio veterum avorum) and inferior to the new revealed truth of Christianity.
Traditional Roman values were essential to the mos maiorum. These include:
- Fides. The Latin word fides encompasses several English value-words such as trust/trustworthiness, good faith/faithfulness, confidence, reliability, and credibility. It was an important concept in Roman law, as oral contracts were common. The concept of fides was personified by the goddess Fides, whose role in the mos maiorum is indicated by the antiquity of her cult. Her temple is dated from around 254 BCE and was located on the Capitoline Hill in Rome, near the Temple of Jupiter.
- Pietas. The Roman attitude of dutiful respect towards the gods, homeland, parents and family was expressed by the word pietas, which required the maintenance of relationships in a moral and dutiful manner. Cicero defined pietas as "justice towards the gods.” It went beyond sacrifice and correct ritual performance to inner devotion and righteousness of the individual, and was the cardinal virtue of the Roman hero Aeneas in Vergil's Aeneid. The use of the adjectival form Pius as a cognomen reflects its importance as an identifying trait. Like Fides, Pietas was cultivated as a goddess, with a temple vowed to her in 191 BCE and dedicated ten years later.
- Religio and Cultus. Related to the Latin verb religare, “to bind”, religio is the bond between gods and mortals, as carried out in traditional religious practices for preserving the pax deorum (“peace of the gods”). Cultus was the active observance and correct performance of rituals. Religious practice in this sense is to be distinguished from pietas and its inherent morality. See Religion in ancient Rome and Imperial cult (ancient Rome).
- Disciplina. The military character of Roman society suggests the importance of disciplina as related to education, training, discipline and self-control.
- Gravitas and Constantia. Gravitas was dignified self-control. Constantia was steadiness or perseverance. In the face of adversity, a “good” Roman was to display an unperturbed façade. Roman myth and history reinforced this value by recounting tales of figures such as Gaius Mucius Scaevola, who in a founding legend of the Republic demonstrated his seriousness and determination to the Etruscan king Lars Porsenna by holding his right hand in a fire.
- Virtus. Derived from the Latin word vir (“man”), virtus constituted the ideal of the true Roman male. Lucilius discusses virtus in some of his work, saying that it is virtus for a man to know what is good, evil, useless, shameful, or dishonorable.
- Dignitas and auctoritas. Dignitas and auctoritas were the end result of displaying the values of the ideal Roman and the service of the state in the forms of priesthoods, military positions, and magistracies. Dignitas was reputation for worth, honor and esteem. Thus, a Roman who displayed their gravitas, constantia, fides, pietas and other values becoming a Roman would possess dignitas among their peers. Similarly, through this path, a Roman could earn auctoritas (“prestige and respect”).
See also 
- Karl-J. Hölkeskamp, Reconstructing the Roman Republic: An Ancient Political Culture and Modern Research (Princeton University Press, 2010), p. 17 online.
- Mos Maiorum, Brill Online.
- Hölkeskamp, Reconstructing the Roman Republic, pp. 17–18.
- Hölkeskamp, Reconstructing the Roman Republic, p. 33.
- Carlin A. Barton, The Sorrows of the Ancient Romans: The Gladiator and the Monster (Princeton University Press, 1993), pp. 176–177.
- Hölkeskamp, Reconstructing the Roman Republic, pp. 33–35.
- Cicero, De officiis 1.35.
- Erich S. Gruen, "Patrocinium and clientela," in The Hellenistic World and the Coming of Rome (University of California Press, 1986), vol. 1, pp. 162–163.
- Suetonius, De Claris Rhetoribus, i.
- See, for instance, Hölkeskamp's reference to the Republic's "capacity for self-regulation," Reconstructing the Roman Republic, p. 18. Erich S. Gruen, The Last Generation of the Roman Republic (University of California Press, 1974), p. 535.
- Hölkeskamp, Reconstructing the Roman Republic, pp. 29, 41–42 et passim.
- Hölkeskamp, Reconstructing the Roman Republic, p. 42.
- Gruen, The Last Generation of the Roman Republic, pp. 258, 498, 507–508.
- The Second Samnite War was a crucial period in the formation of this new elite; see E.T. Salmon, Samnium and the Samnites (Cambridge University Press, 1967), p. 217, and Erich S. Gruen, "Patrocinium and Clientela," in The Hellenistic World and the Coming of Rome (University of California Press, 1984), p. 163 online.
- T.P. Wiseman, Clio's Cosmetics (Leicester University Press, 1979), pp. 67–69, 85, et passim.
- For this pervasive theme in studies of Augustus's political strategies, see for instance M.K. Thornton and R.L. Thornton, Julio-Claudian Building Programs: A Quantitative Study in Political Management (Bolchazy-Carducci, 1989), p. 106 online; E.J. Kenney, The Age of Augustus (Cambridge University Press, 1982), p. 42; The World of Rome (Cambridge University Press, 1997), p. 132 online.
- Clifford Ando, "The Palladium and the Pentateuch: Towards a Sacred Topography of the Later Roman Empire," Phoenix 55 (2001), p. 388.
- Hölkeskamp, Reconstructing the Roman Republic, p. 34.
- “Bona fides,” Berger. pg 374
- Adkins. pg 78
- Ziolkowski, “Temples”
- Adkins. p. 180
- De Natura Deorum. 1.116
- According to Livy, Ab Urbe Condita. xxxx. 34
- Adkins. pg 190
- Adkins. pg 55
- Ward. p. 58
- Ab Urbe Condita. xxii. 58. See also Ogilvie’s Commentary on Livy 1-5.
- Ab Urbe Condita. ii. 12
- Ward. p. 57
- Ward. p. 57
- Ward. p. 58
- Adkins, L. and Adkins, R. Dictionary of Roman Religion. New York: Oxford University Press, 2000.
- Berger, Adolph. Encyclopedic Dictionary of Roman Law. Philadelphia: The American Philosophical Society, 1991.
- Brill’s New Pauly. Antiquity volumes edited by: Huber Cancik and Helmuth Schneider. Brill, 2008 Brill Online.
- Oxford Classical Dictionary. 3rd Revised Ed. New York: Oxford University Press, 2003.
- Stambaugh, John E. The Ancient Roman City. Baltimore: The Johns Hopkins University Press, 1988.
- Ward, A., Heichelheim, F., Yeo, C. A History of the Roman People. 4th Ed. New Jersey: Prentice Hall, 2003.