In Nicaragua before the Sandinista revolution, children who were deaf tended to grow up isolated from one another and the vast majority were born to parents who could hear and speak, but could not sign. Thus, deaf children did not have a sign language model to follow. Most of them developed systems of “home sign,” idiosyncratic systems of gesture that allowed them to communicate with their caregivers and families. Starting in 1977, a central school for the deaf was established, which brought together deaf children from all over the country for the first time in Nicaragua. Children were taught officially using oral methods (speaking and lip-reading training), but they communicated with each other in their spare time using gestures. Early on, the deaf children’s system of gestures developed a standard, shared vocabulary, but the system lacked many of the grammatical features of full-blown sign languages. However, as younger children were added to the mix, they spontaneously added grammatical features found in other sign languages. This was accomplished even though the deaf children were not exposed to an adult language model. One way to explain this phenomenon is to propose that children’s genetic heritage provides them with the mental tools they need to “invent” a grammatical system, as well as the drive to implement such a system if it is not already present in their environment.

—Matthew Traxler, "Introduction to psycholinguistics"