Usage variables usually involve superficial aspects of linguistic structure, but those that are stable and persistent reach deeper into the language faculty. Two grammatical niceties of standard English that are frequently botched even by people who are nominally standard-bearers are Subject-Verb Agreement with dummy subjects (as in There's twelve months in a year for There are twelve months...) and Accusative Case Concord with Conjoined Pronouns (as in Between John and I, we won three games for Between John and me...). Unlike normal variation, the nonstandard variants are not seen as stylistic choices but as mistakes. These usage variables persist not because of failings of the education system but because of the futility of its expectations. The prescribed grammatical forms invoke scope mechanisms that tax human processing capabilities in specific structural configurations. Grammars prescribe forms that the language faculty cannot reliably produce in the multiple tasks involved in ordinary conversation. The discovery of cognitive limitations that override grammatical processing qualifies the strong version of Chomsky's concept of the Language Faculty as an autonomous 'mental organ', and the concept of grammatical processing as hierarchical rather than linear. The persistence of these unstable constructions as grammatical prescriptions reinforces key concepts in variation theory, especially Kroch's concept of standard grammars as ideologically-motivated social constructs.
"‘Bad’ Grammar and the Language Faculty,"
University of Pennsylvania Working Papers in Linguistics:
2, Article 4.
Available at: http://repository.upenn.edu/pwpl/vol16/iss2/4