Sociolinguistics 1, Exercise 3: Martha's Vineyard

Two-sentence summary (as reported by a LINGUIST List announcement of Josey's
2004 dissertation, a second-hand source): Labov found that the first element
of the diphthongs /ay/ and /au/ was centralised, especially by men, notably
middle-aged fishermen. The centralisation correlated with local identity
(heritage) and resistance to summer visitors.

What it suggests: that a sound difference is not random. It carried a social
meaning, so a "change in pronunciation" can be a way of showing who you are.

Caution: this is a secondary account, and Josey's later data show movement
away from centralisation. The primary study is Labov's 1963 work.