The cover of the August 18 print edition of The Economist:
[T]he normal third person singular ending in standard southern English was -eth. The form -(e)s, originally from Northern dialect, replaced -eth in most kinds of use during the seventeenth century. A few common short forms, chiefly doth, hath, continued often to be written, but it seems likely that these were merely graphic conventions.
So why does The Economist use the -eth ending with a plural subject? Well, it's a reference to the motto of New College, Oxford, "Manners Makyth Man", attributed to William of Wykeham (1324 - 1404).
But this just pushes the question back to the 14th century: Why did William of W use a singular verb form with a plural noun? Was -yth /-eth a plural form in his era? Or did he simply choose to construe manners as singular?
Apparently it was the former. According to Goold Brown, The Grammar of English Grammars, 1851:
In early times also the th was an ending for verbs of the third person plural, as for those of the third person singular *…
* The Rev. W. Allen, in his English Grammar, p. 132, say: "Yth and eth […] were formerly, plural terminations; as, 'Manners Makyth man.' William of Wykeham's motto. 'After long advisement, they taketh upon them to try the matter.' Stapleton's Translation of Bede. 'Doctrine and discourse maketh nature less importune.' Bacon." The use of eth as a plural termination of verbs, was evidently earlier than the use of en for the same purpose.
And of course the meaning of the New College motto is not that etiquette is important, but rather that people are defined by how they behave, not by their birth or wealth.
Update — But not so fast. According to Mark Griffith, "The Language and Meaning of the College Motto":
‘Manners makyth man’ is the motto of both the institutions founded by William of Wykeham, New College and Winchester College. Although no contemporary record attests to this, it is often thought that Wykeham devised it himself and appended it to his coat of arms. We do not know when he first used either the arms or the motto, but his seal as Archdeacon of Lincoln displays the arms and he was appointed to this position in 1361. […]
The earliest datable appearance in college is found on the small silver seal which bears the initials of John London, Warden from 1526 to 1542: Manner + Makyth + Man, and nearly the same spelling is found in a stained glass window of c. 1560 in the Warden’s Lodgings: Maner Makyth Man Q[uod] D[icit] Byshop Wykham. A sixteenth century stained glass window on the north side of the nave of Bradford Peverell church, near Dorchester, where the motto appears on a scroll above the arms in the form manare makythe man, represents its earliest known use outside College. This church was given by Wykeham to Winchester College in 1395. These three earliest examples of the motto all display the singular form of the first noun. The standard orthography now found in College, however, is manners. This form is morphologically plural but may have functioned syntactically as singular with collective sense. In southern dialects of Middle English -eth (variously spelled) is the inflection of the third person singular and plural of the present indicative of verbs, but midland dialects characteristically distinguish singular -eth from plural -e(n). The founder, born at Wickham in Hampshire and educated at Winchester, was a southerner and so would have said manner maketh man and manners maketh man. Oxford, however, in the south midlands and bordering southern dialect areas, was situated in an area where both -eth and -e(n) might occur in the plural, so that a local would have said manner maketh man but either manners maketh man or manners maken man (or manners make man). Accordingly, if the current standard form was the original one, then both its subject and verb were ambiguous in number whether its original dialect was southern or south midlands. However, there are pointers to the conclusion that the original form was singular: the singular form manner in the earliest example of the proverb and in the earliest examples of the motto, together with the absence from Wykeham’s foundations of unambiguous verbal plurals in make(n).
Griffith also notes that
Authorities disagree about the motto’s meaning, in particular, the exact sense of ‘manners’. OED ascribes to it in this idiom the sense ‘a person’s habitual behaviour or conduct, esp. in reference to its moral aspect; moral character, morals’, but the MED gives it the more narrowly social sense ‘a way of conducting oneself toward others; outward behavior, deportment, bearing…proper conduct, good manners’ where no explicit link is drawn with moral principles. Both are found quite early in adaptations of the proverb: manner(s) and clothing make(s) man occurs several times in fifteenth century texts and illustrates the superficial interpretation, but grace and manerz make a man in Rawlinson MS C813 from the end of the same century appears to show the deeper or broader sense. In practice, the two are often hard to distinguish in late Middle English, but the founder, a bishop and, by all early accounts, a pious Christian, is unlikely to have selected a motto without strong moral import, still less would he have given such a one to an institution for the education of clerics.