Imagine being a linguist and getting to watch a brand-new human language take shape in real time — not creolised from an existing spoken language, but generated by children in a single generation, starting largely from their own home-sign systems and gestures. That is roughly what happened in Managua, Nicaragua, between 1977 and the early 1990s, producing what we now call Idioma de Señas de Nicaragua (ISN), Nicaraguan Sign Language. Other recently-formed sign languages (Al-Sayyid Bedouin Sign Language in Israel, Kata Kolok in Bali) have since been studied, but ISN remains the best-documented case for cohort-by-cohort emergence.
Before 1977, deaf Nicaraguans were isolated. Each family invented its own "home signs" — small idiolectal gesture systems used between a deaf child and immediate relatives — but the country had no deaf community, no sign language, and almost no education for deaf children. Then, in 1977, the new Centro Nacional de Educación Especial opened in Managua, followed by a vocational school in Villa Libertad in 1981. For the first time, hundreds of deaf children were brought together daily.
The schools taught in spoken Spanish with lip-reading and finger-spelling. Officially, signing was discouraged. The children, however, did what humans always do when forced into proximity: they communicated. On the playground and on the school buses, they combined their home signs, invented new ones, and within a few years had built a shared pidgin sign system, now called Lenguaje de Signos Nicaragüense (LSN).
Then came the revolution. When younger children — preschoolers and early primary kids — entered the system in the mid-1980s, they took the older kids' pidgin as input and did something only children can do: they grammaticalised it. They added consistent spatial agreement, verb morphology, and discourse markers. By the early 1990s, the younger cohort was signing a fully grammatical, fully natural language — ISN — that the older cohort could only partly follow.
The American linguist Judy Kegl was invited to study the situation in 1986. She quickly realised what she was seeing was unprecedented. Together with Ann Senghas, Marie Coppola, and others, she has documented the language across cohorts ever since. Their landmark finding:
"The present study shows that children initially creating the language began analyzing complex events into basic elements, and sequencing these elements into hierarchically structured expressions, following principles not observed in gestures accompanying speech in the surrounding language. Successive cohorts of learners extended this procedure, transforming Nicaraguan signing from its early gestural form into a linguistic system." — Senghas, Kita & Özyürek, Science 2004
The study documents how children in successive cohorts introduced features such as decomposing motion events into "path" and "manner" components, despite those features not being provided by adult signers. These patterns are also documented in other established sign languages.
Today ISN has perhaps 3,000 signers and a Deaf community organised through Asociación Nacional de Sordos de Nicaragua. The first cohort's early signing had to be reconstructed from those signers as adults, but from 1986 onward successive cohorts were recorded on camera and described in linguistic fieldnotes. Those records make changes in the language observable across generations.