Martha's Vineyard deaf population...How did this come to be, please?

Extracted from http://marthasvineyard.com/html/penn/history.htm

"As a result, intermarriage among Chilmarkers brought out a genetic tendency for deafness. At one time more than one-quarter of Chilmark's residents were deaf and could not speak. Old time Vineyarders remember people signing in Chilmark rather than speaking, a difference, not a disability."

There are also records of hearing people on the island using sign language to communicate with other hearing people, even when no deaf were present.

You can also check out the book "Everyone Here Spoke Sign Language: Hereditary Deafness on Martha's Vineyard" by Nora Ellen Groce. Harvard University Press.