From Daniel Nettle and Suzanne Romaine’s Van­ish­ing Voices (via Bren­dan Koerner’s excel­lent Microkhan blog):

Lan­guage is, to adopt the ter­mi­nol­ogy of the French soci­ol­o­gist Pierre Bour­dieu, a form of sym­bolic cap­i­tal that may be as valu­able in its way as are con­crete goods. The tra­di­tional New Guinea sit­u­a­tion makes sense from this per­spec­tive. Larger lan­guages were avail­able to be learned at min­i­mal costs. Indeed, many people knew them already. How­ever people were con­cerned to max­i­mize their social cap­i­tal within their imme­di­ate sur­round­ings. It was, after all, the local group’s ter­ri­tory, and the local group within which one’s family had to exist. There was a great incen­tive to main­tain, along­side any regional lan­guages used for trade, a form of speech pecu­liar to one’s local group which was used within it and which cor­re­lated with a com­mit­ment to it. As William Foley puts it, ver­nac­u­lars were the “indis­pens­able badge of a community’s unique iden­tity.” This factor may well be enough to account for the main­te­nance of so many languages.

(Morobe dis­trict lan­guage map via, sigh, the Summer Insti­tute of Linguistics)