Children learning sign language at Martha's Vineyard, circa 1880. Martha's Vineyard had it's own system of sign language as 1 in 155 people living there were deaf. The national average at the time was 1 in 5,728.

Deafness is usually a recessive gene, yes. Even many of the hearing children could’ve been carriers, then having deaf children. From what I read, deafness can only be caused by recessive & (in less common cases) one copy of a mutated dominant gene.

Martha’s Vineyard is an island and I assume the population started quite small, so even one deaf/hearing mixed family could have quite the genetic impact in that area. Once migration mostly ceased, those families were probably separated from the mainland most of the time. Living on an island where the main population is a boat ride away, they’d probably mostly be having children amongst themselves. I could see how that would explode the population. But it’s very wonderful to read about; deaf people weren’t usually treated as equals back then, but were in this community.

From the link:

“Hereditary deafness had appeared on Martha's Vineyard by 1714. The ancestry of most of the deaf population of Martha's Vineyard can be traced to a forested area in the south of England known as the Weald—specifically the part of the Weald in the county of Kent. Martha's Vineyard Sign Language (MVSL) may be descended from a hypothesized sign language of that area in the 16th century, now referred to as Old Kent Sign Language. Families from a Puritan community in the Kentish Weald emigrated to the Massachusetts Bay Colony in British America in the early 17th century, and many of their descendants later settled on Martha's Vineyard. The first deaf person known to have settled there was Jonathan Lambert, a carpenter and farmer, who moved there with his wife—who was not deaf—in 1694. By 1710, the migration had virtually ceased, and the endogamous community that was created contained a high incidence of hereditary deafness that persisted for over 200 years.”

Astonishing. At least they still managed to have decent lives in many cases.

The high rate of Deafness and signing wasn't really because they were on an island, or at least that wasn't the origin of it. The families who lived slowly migrated overseas together from Kent, bringing their isolated sign language dialect from there. This was from a time when like 90% of most people's lives took place in less than a 20 mile radius.

Deaf people were some of the islands' earliest white settlers. They arrived already signing with their families and friends abled to sign and understand them.