In Chapter 14 I review all the linguistic arguments in support of the Innateness Hypothesis that we have critically examined in this book. I first summarize these arguments. We will then have a number of imaginary speakers (any resemblance to actual people in the field is purely coincidental) reflecting on the Innateness Hypothesis, and on the force of the various arguments pro and con. In this chapter I also briefly preview the arguments that are discussed in a sequel book (Genes, Brains and Evolution: The Language Debate Continued), which critically examines arguments in potential support of the Innateness Hypothesis for language that are based on other disciplines than linguistics proper. In this sequel book, we review how the nature–nurture debate has played out in a number of other sciences, such as psychology, sociology, anthropology, computational science, and a variety of biological disciplines, such as ethology and evolutionary science.

Chapter 1 introduces the central theme of the book, which is in essence the question of how human beings come to know what they know. Trying to answer this question, while considering different views, leads to getting involved in the infamous ‘nature–nurture debate’. I will discuss what kinds of more specific questions we ask when we engage in this debate and also how this debate has changed over time. We then focus our attention on how this debate applies to human language. Specifically, this chapter discusses the Innateness Hypothesis for language, i.e., Noam Chomsky’s idea that children are born with a specific “instinct” to acquire language. This chapter also contains a preview of all the subsequent chapters.

Chapter 5 investigates mental modularity, which is a central concept in the study of minds, i.e., the notion of mental module which, in this context, refers to a specific, specialized domain-specific mental capacity (such as for language, for vision, for music, etc.). A given module may contain several submodules. We will look at the history of this concept and how it has been understood in different approaches, such as the outdated pseudoscience phrenology, the philosopher Jerry Fodor’s nine criteria for proper modules, massive modularity in evolutionary psychology, and other views. Once modules are postulated, we can ask, separately for each module, about the interplay between nature and nurture: Different outcomes are possible for different modules. Finally, we discuss the notion of ontogenetic, developmental modules.

Chapter 8 reveals that languages change over time, with new variants developing and others going obsolete. This chapter aims, firstly, at giving the reader insights into the phenomenon of language change, which has resulted in a wide variety of languages spread throughout our planet that can be grouped into language many families that will be surveyed. We will ask why languages change and provide examples of changes that affect different parts of the grammar, with special attention to grammaticalization. We stress that languages are not getting better or worse as a result of these changes. While we see that under the right circumstances a language can “split” and develop into two or more different languages, attention is also paid to the fact that languages can go extinct. With that general background, we can ask whether the study of language change has resulted in an argument that could support the Innateness Hypothesis for language. To this end, we will ask whether there are certain properties of languages that are immune to change and if so, whether this can be explained if we assume that these properties are anchored in the innate system?

Chapter 10 discusses various characteristics of the overall developmental progression of language acquisition. We first discuss some general properties of this process and then show how it can be studied both with respect to language production and language perception. We discuss the stages and milestones that children go through for different aspects of grammar and ask whether the properties and timing of stages lend support to the Innateness Hypothesis for language. We then formulate the argument from stages. Here the idea is that a complex system like language “unfolds” in the human mind, step by step, each step occurring at more or less regular points in time, as determined by a biological clock. This process of unfolding is called maturation. Just as our body gradually changes into a mature system, so does our mind. This process of unfolding is biologically determined and largely outside the control of the organism, although external factors (“nurture“) play a role. We critically evaluate the argument from stages, asking how precisely it might support the Innateness Hypothesis.

Chapter 6 presents an overview of the organization of the mental grammar. We will focus on general architectural properties of the mental grammar, that is, the units and rules that every grammar must have to capture the sound form, meaning, and syntactic structure of words and sentences. I will suggest that the grammar functions like a checking device in that it tells the language user whether linguistic expressions are well-formed (i.e., grammatical, in accordance with the rules of grammar). There is some technical detail (and many linguistic terms), but at the very least the reader will be left with the conviction that languages are quite complex. It is explained how languages allow people to express any thought they might have, drawing attention to the pivotal notion of recursivity. This chapter sets the stage for being amazed that children have pretty much full control of their language by the age of 4. By learning what a mental grammar might look like, the reader can form an idea of what it is that the child needs to acquire. Without such information, it would be difficult to discuss the role of nature and nurture in language.

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.