Premise: It is quite firm about the r -sound when it follows a vowel-sound or is at the ends of words, not dropping it as is usual with the RP but insisting that it be given full value, in various ways, as it is in Scots and Irish and, generally but not always, in the speech of most Americans except for those in New England and the South.

Hypothesis: Many Americans whose roots lie in Ireland or Scotland still have some of the same speech patterns as their ancestors.
Relation: neutral