Interest in what now are termed the learning disabilities began in the 19^th century with the increased emphasis on universal public education and with an awareness of adults with major problems in reading. This coincided with the move from a less educated to a more educated society. Early work on learning problems in the 1920s speculated that left hemisphere damage (to language centers) was involved.

Modern interest really began with Samuel Kirk who proposed the term learning disabled in 1963. This concept rapidly gained acceptance and the Association for Children with Learning Disabilities was formed. Awareness of children with these problems also led to an understanding that problems sometimes existed in other areas, for example, in mathematic reasoning or in writing rather than only in reading. The passage of Public Law 94-142 (the Education for All Handicapped Children Act) in 1975 specifically recognized learning difficulties as a disability that should be the focus of school-based intervention. Prior to that time, services in public schools were generally limited.

In terms of terminology, it is important to note that in the United Kingdom, the term learning disability refers to children who in the United States would be said to have intellectual disability or what used to be termed mental retardation; in the United Kingdom, the term learning difference is more typically used to refer to what in the United States is termed learning disability. Other terms like dyslexia (for reading problems), dyscalculia (for math problems), and dysgraphia (for writing problems) also have been used.

Children can learn in different ways, and a learning difference does not necessarily mean a child has a learning disorder. These conditions appear to be strongly brain based and have clear genetic as well as important environmental components. Individuals with learning disorders do face a number of challenges not only in school but subsequently in terms of adult occupational functioning. By their very nature, these problems usually come to attention as children enter schools, and in the United States, a multidisciplinary team often evaluates the child to establish the presence of a learning disability and develop an intervention
program. Specific learning disabilities make up about 50% of all special education students (Grigorenko, 2018).

Additional difficulties arise because, in addition to legal definitions (which themselves change over time), somewhat different approaches have been taken by the many different specialties dealing with these problems. Under current law, a child who exhibits a learning disability can qualify for special help and services in school and an individualized educational program is developed once it is clear, following an assessment, that the child qualifies for such services. Changes in the legal definition of learning difficulties can have important implications for service (see Grigorenko, 2018).

In 1980, DSM-III included a concept termed academic skills disorder. This concept has evolved in both DSM and ICD to encompass several different disorders. Both the DSM-IV-TR (APA, 2000) and ICD-10 (WHO, 1994) used an approach based on a discrepancy model, that is, where the child’s performance on an achievement test is significantly lower than IQ. This approach was criticized for various reasons including for being overly stringent and working less well for children with higher or lower overall intellectual ability.

The DSM-5 approach (APA, 2013) continues to recognize three main categories of learning problems in the areas of reading, writing, or arithmetic in school-aged children. These features include the following: (1) persistence for at least 6 months even after provision of a quality evidence-based intervention; (2) achievement testing that confirms that the individual is performing significantly below age-expected levels (the level of service provided should be considered, that is, some children receiving intensive services may improve but still have learning challenges); (3) typically, these problems are observed as children enter school and academic demands increase; and (4) the problems are not due to other factors like intellectual deficiency, absenteeism or lack of schooling, sensory impairment, and so forth. The changes in DSM-5 reflect an awareness of the response to intervention (RTI) approach in assessment and treatment of learning difficulties (Bradley et al., 2005; Denton, 2012) as well as the awareness of changes in federal mandates for schools to provide services. It should be noted that other types of learning problems, for example, nonverbal learning disability, may fall outside the DSM-5 definition but still present challenges for learning.

Data on rates of learning disorders come from two sources—school reports of students receiving mandated services and more rigorous epidemiologic research. For school-based learning problems (see Figure 9.1), this typically ends up with estimates of 5% to 6% of school-aged children in the United States. Reading problems account for 75% to 80% of cases. It is important to recognize that rates vary markedly from school district to school district and state to state, reflecting continued differences in the approach to assessment and diagnosis of these problems (Grigorenko, 2018). Boys are twice as likely to have been recognized as having learning disability. It does appear that girls are more likely than boys to have mathematic learning problems. Rates in minority groups may be underestimated.

Research data with more rigorous and standardized case assessments show that around 1.2% of school-age children have academic difficulties; this number is reduced when schools provide high-quality teaching. Much of the available research has focused on reading difficulties and less is known about other learning problems. Changes in approach to diagnosis have influenced these estimates. Learning problems are frequently related to issues of attention, and attention deficit disorder is a common comorbid condition.

Genetic and neurobiological factors as well as experience play a role in the pathogenesis of learning difficulties. A large body of work has now underscored the importance of genetic factors in these conditions (Becker et al., 2017; Grigorenko, 2018; Soares et al., 2018;). Environmental factors, including prenatal ones, are important as well (Becker et al., 2017; Mascheretti et al., 2018). It appears that all these risk factors act through one or more brain mechanisms that are important in reading, writing, and mathematics skills. Extreme deprivation and lack of opportunity as well as poverty may also contribute to learning problems. Other difficulties like head trauma and treatment of childhood cancers (e.g., CNS radiation) can be implicated.

Most of the work on etiology has centered around specific reading disabilities. Various methods have been used to study the neurobiologic correlates of reading problems (Becker et al., 2017; Grigorenko, 2018). These have included MRI, EEG, genetic association studies, and so forth. As might be expected, reading skills involve a large network of brain areas predominately in the left hemisphere of the brain once they are fully established (see Figure 9.2). These involve brain regions involved in visual and auditory as well as more basic conceptual processing. The four areas that appear to be of greatest relevance to reading include the fusiform gyrus (Brodmann area (BA) 37), a portion of the middle temporal gyrus (including portions of areas 21 and 37, along with the angular gyrus (BA 39), and the posterior aspect of the superior temporal gyrus (BA 22)). Differences in brain activation have strong developmental correlates and more able readers seem to shift to frontal regions, whereas those with difficulty tend to use the most posterior ones (in the parietal and occipital regions).

There are some developmental changes in brain processing as academic skills become more well established. Over time, there is a progressive disengagement of the right hemispheric areas. In addition, there are some shifts in regional activation preferences, with frontal regions less involved over time.

Since the 19^th century, there was an awareness that specific reading difficulties did, at times, strongly run in families. This has become an even more active area of investigation with the
availability of more specific genetic approaches and advances in techniques of gene identification. A range of approaches have now been used including twin, sib pair, and family studies, and it now appears that multiple genes are likely involved (Grigorenko, 2018). Sites have been identified on chromosomes 15q, 6p, 2p, 6q, 3cen, 18p, 11p, 1p, and Xq; efforts are now underway to identify specific candidate genes (Riva et al., 2019; Sanchez-Moran et al., 2018). Several genes have now been identified (for more information, see Foster et al., 2015; Grigorneko, 2018).