This chapter covers the cognitive underpinnings of dyslexia and dyscalculia and their co-occurrence, from the perspective of the multifactorial deficit model of developmental disorders (Pennington 2006). The two disorders are first defined with respect to their typical behavioural manifestation. Next, prevalence rates of single as well as co-occurring deficits are described and critically discussed in light of methodological challenges. Subsequently, the cognitive bases of each disorder, as well as their comorbidity, are specified in the light of the multifactorial deficit model. Finally, methodological considerations regarding the link between dyslexia, dyscalculia, and other neurodevelopmental disorders are presented. Future avenues of research are also outlined, with a particular emphasis on treating reading and maths performance as a continuous variable, and on longitudinal studies. The studies presented in this chapter are mainly based on children attending primary school (age range approximately 6–12 years old). There are, of course, differences in the organization, duration, and onset of formal instruction across countries. Aside from nation-specific differences, however, this age period is particularly relevant for research on dyslexia and dyscalculia, as this is the typical time window when literacy and arithmetic skills are acquired.

1. Causation vs. correlation: Longitudinal observation and particularly intervention studies beginning in infancy (which are currently scarce) are the key to understanding the causes of learning disorders.

Why do some children arrive in kindergarten already knowing how to add, while others struggle to count? Why do some students avoid challenging maths problems, while others embrace them? Given the importance of maths learning for academic success and future earning potential (e.g., Ritchie and Bates 2013), and the strong relation between school-entry maths skills and later maths achievement (Watts et al. 2014), it is critical to understand the sources of variability in maths knowledge in early childhood.

Education is a major pillar of contemporary society. Educating children provides them with stability, purpose, and financial security at an individual level and bolsters economic growth at a societal level (Hanushek and Kimko 2000; Jamison et al. 2007). Nonetheless, in the developing world, more than 200 million children under 5 years of age live in poverty, with poor nutrition, limited access to healthcare, and either inadequate or no education (Engle et al. 2007). Universal education provides a key pathway towards societal equality. UNICEF Canada suggests that education is ‘a powerful tool for breaking the cycle of poverty; supporting child survival, growth, development and well-being; and closing the gap in social inequality’ (Unicef Canada 2016).

Neuroplasticity refers to the ability of the brain to reorganize itself following experience and, as such, it represents a foundational milestone for any type of learning. At the microscopic level, neuroplasticity can be described via modifications of neuronal morphology or changes in neurobiochemical balance (Fuchs and Flügge 2014). These microscopic changes, in turn, can lead to macroscopic alterations in the morphology or functionality of certain brain regions (i.e., structural and functional changes, respectively), and to circuit reorganization, at the regional or network levels (i.e., connectivity changes), as a function of learning.

(1) Learning challenges of alphabetic scripts: Learning English is most difficult since it is least transparent in terms of print-to-speech conversion (and vice versa) and most complex in terms of syllable structure, while Italian shows one of the most extreme opposite patterns.

Understanding the neurobiological underpinnings of learning difficulties, particularly developmental dyslexia, has received substantial attention since functional MRI and diffusion MRI first became widely available in the 1990s. Now, a couple decades later, there are hundreds of studies documenting differences in brain structure and function in people with dyslexia. Despite vast differences in educational systems, writing systems, and languages around the world, and differences in methodologies and samples across labs, there are at least three points that are broadly accepted.

(1) The ‘big three’ predictors of dyslexia in alphabetic learning contexts: Phonological awareness, letter knowledge, and rapid automatized (object and symbol) naming most strongly predict reading outcomes and dyslexia (explaining 40–60% of the behavioural variance). In scripts with transparent print-to-speech conversion, phonological awareness most strongly predicts reading outcomes during early stages of learning (but remains a reliable predictor in later grades for spelling).

Dyslexia and dyscalculia originate from the joint effects of multiple genetic and environmental factors on the developing brain in the first years of life, including the prenatal phase. While many candidate genes have been reported in the literature (e.g. DCDC2, DYX1C1, KIAA0319, and ROBO1), so far no DNA variants have revealed reliable associations across the whole genome. Moreover, potential early environmental influences (e.g. preterm birth, prenatal substance exposure, breastfeeding, maternal psychosocial stress, developmental toxicants) currently preclude specific interpretations since they also have a genetic basis that is typically not controlled in existing studies. Correspondingly, the question of how gene–environment correlation and interaction lead to learning difficulties can only be answered at a very general level at present.

(1) Fostering learning beyond learning disorders: All children who struggle with learning (including children with underattainment or subclinical difficulties, but without a dyslexia or dyscalculia diagnosis) need measured support from the education system following an explicit protocol of referral and response (e.g., primary school systems in Norway and Botswana). This is particularly important in global contexts given that many children worldwide have limited or even no access to high-quality and contextually appropriate learning opportunities, and also in heterogeneous contexts in which children, for example, do not know the language of instruction well because it is a new language or a new dialect.

Children typically record dramatic growth in reading skills in primary school. This is not surprising since reading instruction has been placed at the heart of primary school curricula. Irrespective of country, learning outcomes in primary schools everywhere articulate the expectation that children will read, comprehend, and write to express themselves well. This chapter focuses on children who fall behind these stated literacy expectations within their school system, arguing that, while some will qualify for the diagnosis of dyslexia, all those who struggle will need a measured response from the primary school system. A cursory look at the decision-making framework that primary schools follow for those who fall behind shows variation across countries and also within countries, including those that have a well-articulated protocol of referral and response. Given this, it is useful to think of school systems as placed at different points on a continuum of support, with schools that have no discernable provisions and schools with multilayered provisions put at either extreme. This variety may reflect differences in access to resources for screening, diagnostic testing, and tiers of targeted interventions.

Persistent, specific, and unexpected difficulties in the acquisition of efficient reading and/or spelling abilities (dyslexia) and mathematical abilities (dyscalculia) leads to marked impairment in school, at work, and in everyday life. Furthermore, years of academic frustration can have the effect that many affected children and adolescents encounter problems such as low self-esteem and symptoms of anxiety and depression (Carroll et al. 2005; Visser et al. 2020). The development, evaluation, and implementation of effective remedial interventions for children with dyslexia and dyscalculia is therefore of profound importance for mitigating the negative consequences for the life of affected children.

In the simplest sense, reading acquisition is a process of learning how a writing system represents a spoken language. It requires conscious awareness of how the basic graphic symbol, the grapheme, represents a spoken unit. However, unlike language acquisition, which occurs naturally, rapidly, and automatically in most children, learning to read is a lifetime developmental process that requires effort, explicit instruction, and conscious awareness of the language structure (known as metalinguistic awareness). Many children fail to achieve full proficiency in reading, and some exhibit specific problems with reading, writing, and spelling despite adequate intelligence, motivation, and educational opportunity. This specific learning difficulty, known as developmental dyslexia, has varying incidence rates depending on the diagnostic criteria and the language examined. The divergent incidence rates, together with some findings suggesting that dyslexia has different behavioural and neural manifestations within and across cultures, has led to the question of whether the underlying causes of dyslexia differ across writing systems and whether it is possible to be dyslexic in one language and not dyslexic in another.