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.

We have seen that in normal cases children create their mental grammar from input that has been called “poor,” but that language fails to emerge when input is totally absent. Chapter 12 shows that when the input is of “extra-poor” quality, children nevertheless manage to arrive at mental grammars of apparently sufficient complexity. The relevant situation can lead to the emergence of whole new languages. The key claim is that children, when exposed to a so-called pidgin (a relatively primitive system of communication created by adults who do not share a common language), transform such an impoverished language into a full-fledged language, which we call a creole. How can children do this? Nativists have argued that children can do this because they have an innate “manual” that dictates the construction of a mental grammar, no matter how impoverished the input. This leads to the creolization argument in support of the Innateness Hypothesis. This chapter also surveys various kinds of artificially constructed languages that are typically created by adults who either want to design a universal language or to provide groups of people in fictional stories with their own language.