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 We will look at the history of this concept and how it has been understood in different approaches, such as the outdated pseudoscience phrenology, the philosopher Jerry Fodor’s nine criteria for proper modules, massive modularity in evolutionary psychology, and other views.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.

Chapter 4 describes the field of cognitive science, which is the arena where all those who study “intelligent systems” (“minds“) get together to compare notes. A shared idea is that the mind can be understood as an information-processing computational system. We will see how during the 1960s renewed interest in the mind from different academic disciplines emerged as a reaction to the denial of the mind of an approach to psychology called behaviorism. We then discuss the various strands of thinking in a variety of different fields that led to this “cognitive revolution.” We learn that there are fundamental, opposing views in this field that are relevant to the nature–nurture debate. Despite differences, a general understanding within cognitive science is that the mind can be studied at different levels of abstractness and from different angles which to some extent compete but also complement each other.

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.

Chapter 7 evaluates the force of a first argument in favor of the Innateness Hypothesis: the argument from universals. We will distinguish various types of universals, and examples will be provided. We will first look back at the organization of the mental grammar and ask which parts of that system could be innate. It is then made clear that we need to critically examine when alleged universals can be safely used to support the Innateness Hypothesis. We learn that the argument from universals has to be applied with care and without falling into logical fallacies. We need to realize that alleged universal properties of languages may, firstly, be applicable more generally to cognitive systems that include language (in which case they are not language-domain specific) and, secondly, be caused by factors that have nothing to do with the proposed innate Universal Grammar that nativists postulate. To use a universal in support of the Innateness Hypothesis, it needs to be specific to language and not be explainable in terms of other factors. We also see how Chomsky’s ideas about what might be innate for language have changed over time.