Terms such as learning disorder and learning disability often are used interchangeably, although the latter term is used more commonly. A major stride in the definition of learning disabilities came from the National Joint Committee for Learning Disabilities. The National Joint Committee for Learning Disabilities defined a learning disability as a generic term that refers to a heterogeneous group of disorders manifested by significant difficulties in the acquisition and use of listening, speaking, reading, writing, reasoning, or mathematical abilities. These disorders are intrinsic to the individual and presumed to be due to central nervous system dysfunction. Even though a learning disability may occur concomitantly with other handicapping conditions (e.g., sensory impairment, mental retardation, social and emotional disturbance) or environmental influences (e.g., cultural differences, insufficient/inappropriate instruction, psychogenic factors), it is not the direct result of those conditions or influences (Hammill et al. 1981, p. 336).

This definition went further than the earlier one contained in the Education for All Handicapped Children Act of 1975 (P.L. 94–142) by stipulating specifically that a learning disability must be presumed to be due to central nervous system dysfunction. Although this was implied in previous definitions, never before was it made explicit. Accordingly, the newer definition helps to resolve a good deal of confusion and ambiguity involving identification and differential diagnosis. Deficiencies in academic achievement can arise from a variety of factors, operating alone or in combination. To say that there is a learning disability, however, means that there must be a basis for inferring that some form of brain dysfunction is involved.

Diagnostic Criteria

Diagnostic and Statistical Manual of Mental Disorders, 4th edition, Text Revision (DSM-IV-TR) identifies three types of learning disorders: (1) reading disorder, (2) mathematics disorder, and (3) disorder of written expression. In each disorder, the diagnosis depends on documentation that

• Achievement in the area, as assessed on an individually administered standardized test, falls substantially below expectations based on the person’s age, measured intelligence, and education
  
• The deficiency significantly interferes with academic achievement or daily activities requiring the particular skill
  
• The skill deficiency exceeds what usually would be associated with any sensory deficit, if present

Inherent in these criteria are considerations pertaining to the severity, extent, and specificity of the observed deficit—whether it be in reading, mathematics, or written expression. The deficit must be nontrivial or substantial, although this is not defined further by DSM-IV-TR. It must affect relevant aspects of daily functioning. It also must be specific and not simply reflective of intellectual, sensory, or educational limitations.

The diagnostic category of learning disorder not otherwise specified refers to deficiencies in reading, mathematics, or written expression that interfere with academic achievement but do not meet criteria for a specific learning disorder. This is an ambiguous category that probably should not be viewed as representing a disorder. At best, it implies that a learning disorder is suspected but cannot be documented through ordinary means.

Epidemiology

Estimates have varied, but about 2–8% of all school-aged children in the United States are thought to have a learning disability. The estimates are arbitrary to some extent, as they are based on the adoption of an agreed-upon cutoff in a continuous distribution. These estimates are influenced not only by debates over what the objective criteria for identification should be (see below) but also by public policy considerations having to do with the allocation of special services.

Etiology

The issue of etiology is addressed, at least in broad terms, by the National Joint Committee for Learning Disabilities definition of learning disabilities. In one form or another, brain dysfunction is the source of a learning-disabled individual’s deficit(s) in reading, mathematics, or written expression. The dysfunction may stem from genetic or congenital factors, arising especially during middle to later stages of fetal brain development. Neuropathologic studies suggest the presence of relatively subtle irregularities (e.g., focal dysplasia, abnormal cortical layering, polymicrogyria), often clustering in the left perisylvian region, although the precise pattern will vary with the type of learning disabilities involved. This observation accounts for the specific nature of learning disabilities, in that earlier or more widespread abnormalities in brain development typically would give rise to more generalized disorders such as mental retardation. Insults occurring after birth may be a factor, provided that they affect the acquisition rather than loss of a particular skill. Although similar deficits may arise, the convention is to regard a learning disability as a neurodevelopmental disorder rather than as an acquired brain injury.

There have been a number of misconceptions regarding the cause of learning disabilities. One misconception has suggested that children with disabilities are not disordered but rather delayed on certain developmental dimensions, their difficulties presumably reflecting a slower rate of maturation of an otherwise normal brain. This would explain why a fairly severe disability can exist in the absence of documented brain impairment, at least when gross indices are used. However, current research does not support such a hypothesis. There is no evidence that the brains of learning-disabled individuals are immature, or unfinished, in some way. Rather, newer and more detailed investigations have documented specific structural abnormalities. Nor is there any indication that the disabled learner’s performance resembles that of a normal younger child or that the disability is eventually outgrown. The child with learning disabilities is not merely delayed but rather deviant in the performance of processes necessary for normal reading, math, or writing. The disability may be “silent” in earlier years, giving the false impression of normal brain development, only to become evident when the child enters school.