Where did pluralis-ut-singularis come from in Latin?

Question

Anyone who reads Cicero's letters cannot fail to notice that he quite frequently uses nos and noster to mean ego and meus. Earlier I heard a paper where nos in Lucretius' proem was meant singularly (primarily because Venus in Epicurean thought cannot have actually helped Lucretius like the Muses help epic poets).

This made me wonder where this plural actually comes from. Is it just a stylistic development? Is its use in English as the pluralis maiestatis derived from Latin? Wikie also suggests its use in other languages, but they're usually fairly traceable developments; Andreas Katsouris 1977 ("Plural in place of singular," RhM 120: 228–240; he suggests that the use of it in Roman comedy may have come from Greek translations, but provides nothing further than that) examined the phenonmenon in Greek, but what of Latin? Is it related at all to the poetic plural?

In case you're not entirely sure about its usage, see Bradley's Lessons in Latin Prose p. 212 (with cleaned up commas):

Obs. 1. Nos, noster, or a plural verb are often used when only one person is spoken of in preference to ego, meus, or a singular verb. This idiom is rare in narrative and in historical writers. Instances of it abound in Cicero, especially in those parts where he is writing in a philosophic, conversational, or epistolary style. Multis de rebus scripsimus, I have written on many subjects.

Answer 1

There are three commonly recognized types of nosism, in which the plural first-person pronoun is used rather than the singular: the pluralis societatis ("social plural"), pluralis modestiae ("plural of modesty"), and pluralis maiestatis ("plural of majesty" or "royal we"). In some sense all of these can be considered specialized uses of the poetic plural, which occurs frequently in the poetry of Classical Latin, as indicated by Gildersleeve & Lodge, §204.6:

The Pl. is freely used in poetry and in later prose: Ōtia sī tollās, periēre Cupīdinis arcūs, Ov., Rem. Am., 139; if you do away with holidays, Cupid's bow (and arrows) are ruined.

The poetic plural appears widely in Ancient Greek as well. Horace Leonard Jones identifies several examples of all three types of nosism (societatis, modestiae, and maiestatis) in the Iliad and Odyssey, and argues that they logically develop from one another:

Three uses of the first person plural pronoun for the singular may be recognized, which apparently represent the logical development of the plural as meaning one person out of the true plural. The steps are: (1) Pluralis Societatis, (2) Pluralis Modestiae, (3) Pluralis Maiestatis. We may suppose that at first the ημεις [we] associates in thought others with the εγω [I]—the speaker, and that the plural is really κατα συνεσιν; the speaker then with purpose sinks his identity in the class to which he belongs, and the notion of modesty, humility results; as in Latin. (The Poetic Plural of Greek Tragedy, 127ff.)

Gildersleeve & Lodge, §204.7, find the practical use of pluralis modestiae to often reflect a false modesty:

The usage [of first person plural for first person singular] originates in modesty, but mock modesty is the worst form of pomposity. It is never very common, and is not found before Cicero: Librum ad tē dē senectūte mīsimus, C., Cat. M., 1, 3; we (I) have sent you a treatise on old age.

Harm Pinkster, Oxford Latin Syntax, v. 1, 11.122, agrees with this analysis and confirms the timeline:

The use of the first person plural by a speaker or writer to refer to his own activities is found from the Classical period onwards [...] A common Latin term for this use of the plural is pluralis modestiae; however not all instances do reflect modesty.

Answer 2

In addition to Nathaniel's excellent answer, we offer this quotation from Ennius:

Liber VII

Ennii de Naevio sententia:

     scripsere alii rem               231 

 Versibus quos olim Faunei vatesque canebant; 

 Cum neque Musarum scopulos quisquam superarat 

 Nec dicti studiosus erat, 

     ante hunc 

 Nos ausi reserare . . . .

This appears to be about how Ennius ("nos") dared open the doors/fount of creativity to compose his poems, or something like that. Kühner–Stegmann pointed us to Ennius.

According to K–S, "we" for "I" is rare in Greek prose but more frequent in Greek poetry, so we wouldn't be surprised if that's where the Romans took it from.

They explain the plural as "modest" in that the author does not wish to call too much attention to himself but rather suggests a certain point of view is held by others as well.

They also state that the modest plural stands in an unmistakable relation to the "general plural". They also mention the plural of "relatives", which appears to be a subspecies of the general plural. From the Aeneid, 2, 579:

(Helena) ... patres (= Tyndarum) natosque (= Hermionen) videbit.

So it says that Helena will see her parents and children upon her return, when in fact she will only see Tyndarus and Hermione. Hermione was her only child, and her mother Leda would according to K–S not be present in Sparta or Mycenae.