1952: From the 18th until the 20th century, the population of Martha’s Vineyard, off the coast of Massachusetts, included a disproportionately large number of deaf people. Isolated farming and fishing communities, and consequent intermarriage, ensured that the defective gene passed from generation to generation.
In the 19th century, when the national average in the United States was one deaf person in roughly 6,000, the figure for Martha’s Vineyard was one in 155. The concentration of deaf people was greatest at the western end of the island, the up-Island; in Chilmark, one in 25 was deaf.
With such a high proportion of deaf people, all residents acquired some fluency in the island’s distinctive sign language. Hearing people would sign even when deaf people were absent; schoolchildren signed behind their teachers’ backs; adults signed to one another during church sermons.
In time, the island’s communities made greater contact with the mainland and that led to a decrease in the incidence of deafness. The last member of the up-Island deaf population, Abigail Brewer, died in early 1952.
Source: Nora Ellen Groce, Everyone Here Spoke Sign Language: Hereditary Deafness on Martha’s Vineyard (1985), pp. 3, 42, 64–5, 94