Almost 150 years after Kussmaul’s documentation of ‘Wortblindheit’ (word blindness) (Kussmaul 1877) the scientific community has generated a number of different theories of dyslexia and dyscalculia. While these theories are still controversially discussed, the converging findings of longitudinal developmental research now allow us to draw an increasingly clear picture of the potential origins of these learning difficulties. At the same time, the common understanding of dyslexia and dyscalculia is blurred by persistent myths, such as the notion that dyslexia causes letters to appear out of order or that dyscalculia is a sign of reduced intelligence. Moreover, families, educators, and even specialized practitioners are often not sure how a specific learning disorder is validly diagnosed and which type of support children need to cope with their difficulties. Accordingly, the purpose of this handbook is to provide a developmentally grounded perspective on these topics by integrating findings from the life sciences and social sciences.

Reading and maths ability are shaped by an interaction of genetic and environmental factors. Roughly half of the behavioural variance that is measured when assessing reading and maths performance is explained by genetic variance, while the other half is explained by environmental variance (see Chapters 6 and 7). In this context, it is often overlooked that there is a large explanatory gap between what genes do and how behavioural performance is regulated. Genes do not act directly but indirectly on reading and maths ability, most notably by playing a role in the development of the brain, the biological information processing system that makes learning to read and do maths possible. More specifically, many candidate genes for dyslexia and dyscalculia encode protein structures in developing neural circuits that ultimately form the brain systems underlying reading and maths. This explanatory pathway involving genetic variation, brain systems for reading and maths, and learning difficulties in these two domains is the key topic of the present chapter.

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.

(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.

(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.