This chapter examines sign languages and shows that they are characterized by the same non-linear principles as spoken/oral languages. It is shown that reflexives in sign languages work analogously to reflexives in spoken/oral languages, and in a way that is more complex that is strictly required by the gestural nature of sign languages.

Sign languages, the languages used by and among deaf people, have long been misunderstood and undervalued. Chapter 13 shows what they really are: human languages. First, we have to rid ourselves of various misconceptions about sign languages. I then formulate the sign language argument for the Innateness Hypothesis, which is based on various parallelisms between signed and spoken languages that strongly suggest that, despite operating in completely different sensory channels, both are likely instantiations of the same mental language system. Both types of languages are processed in the same brain areas and show similar developmental patterns during acquisition and language breakdown. This supports the idea of a genetically anchored default language function for these brain areas. In support of this idea, sign language studies also provide us with examples in which grammatical structure emerges spontaneously when deaf children grow up without being exposed to a sign language. These so-called home sign systems can even give rise to new sign languages. This adds the argument from spontaneous emergence to our list of arguments that potentially support the Innateness Hypothesis.