Learning Disabilities: A Nosology

Learning Disabilities: A Nosology

G. Emerson Dickman, J.D.
Jan. 20, 2003

A nosology is a systematic classification of diseases. A taxonomy is a system for classification usually in rank order. The most recognized example of an inclusive, pyramidal taxonomy is the Linnean Hierarchy (Kingdom, Phylum, Class, Order, Family, Genus, Species). Each level is a taxon, the plural of which is taxa. In 1994 S.E. Shaywitz et al. encouraged “the development of a unitary, empirically derived nosology . . . [that] should increase the consistency and generalizability of findings across investigations and across disciplines.” It should be noted that although the highest-level taxon in this discussion is Learning Disability, it is understood that Learning Disability is only one of several taxa at the same level in a more inclusive classification system. For instance, the level at which Learning Disability appears in a more inclusive nosology (perhaps it could be called Disorders of Brain Function) that might also include Developmental Disabilities, Traumatic Brain Injuries, Mental Illness, and the like.

1. Learning Disability (LD)
2. Classification
3. Specific Learning Disability (SLD)

a. Cognitive Deficit
b. Performance Deficit
c. Manifest Disability
d. Derivative Impact


1. Learning Disability (LD)
2. Classification: Language-based
3. Specific Learning Disability (SLD): Dyslexia

a. Cognitive Deficit: Phonological
b. Performance Deficit: Decoding, fluency, spelling
c. Manifest Disability: Reading
d. Derivative Impact: Comprehension

The taxa at the level of SLD are often seen as descriptive of the manifest disability involved, e.g., Dyslexia (reading), Nonverbal Learning Disability (deficient ability to process nonverbal communications – “elements that are actually more critical to the meaning of a conversation than the words themselves.” However, in order for a SLD to be worthy of scientific consensus as to its validity it must be capable of being defined in terms of the four key elements (1) cognitive deficit, (2) performance deficit, (3) manifest disability, and (4) derivative impact.

In other words, if a Cognitive Deficit does not reliably predict a Performance Deficit, if a Performance Deficit does not reliably predict a Manifest Disability, or if a Manifest Disability does not reliably predict a Derivative Impact, no Learning Disability is present. If intervention or remediation successfully ameliorates the Performance Deficit, thus, diminishing the pernicious effect of the Manifest Disability and Derivative Impact, the Learning Disability continues to exist, in spite of the improvement in adaptive functioning. An apt analogy might be to the circumstance where diet restrictions ameliorate the impact, but do not cure diabetes.

Within the construct of Learning Disability, Derivative Impact is intended to describe a side effect that may be consequential to the Manifest Disability involved, but cannot be predicted by reference to Cognitive or Performance Deficits alone. The connection between the Cognitive Deficit, Performance Deficit and Manifest Disability is lineal, explicit, and causal; whereas, the consequential relation between the Manifest Disability and the Derivative Impact is incidental. In other words, the comprehension difficulties of a child with dyslexia may be due to not reading, as compared to the inability to read. As a result of the lack of exposure to print, the individual fails to acquire the background knowledge and vocabulary necessary for the efficient comprehension of age appropriate text and literature. A derivative impact on comprehension is not directly related to a cognitive deficiency, but, more accurately, to the lack of a sufficiently enriched environment to permit it to fully develop. The cognitive prerequisites to comprehension are unimpaired.

Unfortunately, the Derivative Impact of Learning Disabilities is a potentially endless chain of incidental cause and effect that has a human, social, and political cost. It doesn’t take a great deal of imagination to extrapolate a continuum of negative effect (e.g., underachievement due to low self esteem due to academic failure due to difficulty comprehending and so on). A universally accepted understanding of the concept of Learning Disabilities is essential if research and practice are to join forces to address the needs of individuals with Learning Disabilities, which is recognized as “not just an education problem, [but a] social and health problem as well.”

It is now widely recognized that Learning Disabilities are not just a problem faced by a small number of afflicted individuals, but an occurrence of sufficient magnitude to pose a social and public health problem. As such, it may be helpful to address the study of Learning Disabilities, in all of its manifestations, from an epidemiological perspective.

Impediments to consensus on proposed subtypes of learning disabilities (Specific Learning Disabilities) is often two fold:

  1. The subtype proposed is a symptom complex made up of a recognizable pattern (constellation) of numerous Cognitive Deficits. See the various labels under which the proposed phenotype of a Nonverbal Learning Disability is addressed, supra. In this case, symptom complex refers to a pattern of comorbidity that occurs with sufficient regularity to be recognized as an individual phenotype. A Specific Learning Disability being an individual star, a symptom complex being Orion. I refer to such a proposed Specific Learning Disability as a complex phenotype.
  2. The subtype proposed may be the result of one of several distinct and unidentified cognitive deficits that each result in a similar appearing Manifest Disability, e.g., Dyscalculea (math) Dysgraphia (handwriting).

I refer to a proposed Specific Learning Disability reflecting a single cognitive deficit as a unitary phenotype; a proposed Specific Learning Disability reflecting a pattern of cognitive deficits as a complex phenotype; and a proposed Specific Learning Disability that may be the result of more than one cognitive etiology as a variable factor phenotype.

The identification of a Specific Learning Disability for research purposes requires a phenotypic profile that includes the identification, at least hypothetically, of the Performance Deficit and Cognitive Deficit related to the particular Manifest Disability. For instance, there are many causes for an individual’s inability to read. The phenotype that we refer to as Dyslexia involves an inability to read (Manifest Disability) that is due to a difficulty in decoding, fluency, and spelling (Performance Deficit) as a result of a

Cognitive Deficit in Phonological Processing. Studies of brain function, brain anatomy, and genetic predisposition would be of little value without a well-defined phenotypic profile.

From a functional epidemiological perspective, a Specific Learning Disability could be identified for research purposes by investigating social and public health consequences (Derivative Impact). For instance:

1. Derivative Impact with social and public health implications: drop out rate.
2. Derivative Impact: External Locus of Control (Learned helplessness).
3. Derivative Impact: Cognitive Dissonance (uncomfortable psychological state)
4. Derivative Impact: Expectations exceed performance (incompatible beliefs in self efficacy).
5. Manifest Disability: Difficulty making and keeping friends — social isolation.
6. Performance Deficit: Paralinguistic skills.
7. Cognitive Deficit: Visual perspective taking.


Such a profile can lead to brain and genetic research and the development of scientifically validated approaches to developing strategies for intervention and remediation.

The reason the research relating to Dyslexia is so widely recognized, replicable and reliable is that it is also “the best defined of all learning disabilities.”

Dyslexia in a nutshell:

  • Cognitive Disability = phonological
  • Performance Deficit = decoding, fluency, spelling
  • Manifest Disability = reading
  • Derivative Impact = comprehension, etc.

My experience, unscientific and influenced by significant referral bias, has resulted in my categorizing three main Classes of Specific Learning Disability (SLD):

  1. Language-Based Disorders,
  2. Disorders of Executive Function, and
  3. Disorders of Social Communications.

Clearly, some of the observable manifestations of each SLD are very similar in many aspects. For instance, problems with comprehension may be a Derivative Impact in the case of a Language-based Subtype such as Dyslexia, but may be a Manifest Disability in the case of Disorder of Executive Function, or a Performance Deficit in the case of a Disorder of Social Communications.

Typical textbook descriptions of phenotypic manifestations (meaning observable behavior associated with the disorder) describe the Manifest Disabilities associated with the Class or Specific Learning Disability identified. For instance disorders within the Class of Executive Functioning are described as involving difficulty “initiating action, organizing, planning, self-regulating, maintaining attention, and inhibiting behavior.” The SLD often referred to as the Nonverbal Learning Disability is described as involving “poorly developed organization skills, difficulty making inferences and reasoning abstractly, significant problems with mathematical reasoning, and limited social competence.” Research addressing a similar profile of symptoms appears under the following additional labels: Right Hemisphere Dysfunction, Semantic-Pragmatic Learning Disability, Nonverbal Right Hemisphere Learning Disability, Social Communications Spectrum Disorder, Social Perception Disability, Nonverbal Perceptual Organization Output Disability, Left Hemi Syndrome, Right Parietal Lobe Classification, Nonverbal Social Learning Difficulties, Social-Emotional Learning Disability, even Vestibular Dysfunction and Disorders of Sensory Integration.

If anomalous adaptive functioning is observed (Manifest Disability), e.g., limited social competency, oppositional appearing behavior, the inability to read, it is necessary to analyze antecedent performance and cognitive deficits before a SLD can be identified. For instance: (1) A child that is having difficulty reading has Dyslexia if the problem decoding is due to an unexpected cognitive deficit in phonological processing. (2) A child may have a Disorder of metacognition (in this case, being able to efficiently apply the knowledge that an individual may possess), within the Class of Executive Function, who reads well and has difficulty with expressive writing due to unexpected cognitive deficits in processing information hierarchically and sequentially. (3) A child that appears oppositional due to unexpected cognitive deficits involving perspective taking and processing paralinguistic communications may have a SLD within the Class of Social Communications.

1. Shaywitz, S.E., Fletcher, J.M., & Shaywitz, B.A. (1994). Issues in the definition and classification of attention deficit disorder. Topics in Language Disorders, 14(4), 1-25, at p.22.

2. Jensen, Brain-Based Learning p. 17

3. Testimony before the House Committee on Education and the Work force by Robert Pasternack, Assistant Secretary for Special Education and Rehabilitative Services U.S. Dept. of Education, June 6, 2002.

4. In this case a study of diseases having a public health impact with a focus on the source and causes.

5. Derivative Impact often consists of a hierarchy of incidental consequences related to the Manifest Disability. For instance in Dyslexia the lack of exposure to print (not reading) a higher order Derivative Impact may lead to inadequate “growth of vocabulary and background knowledge” that, in turn, leads to difficulty with comprehension that leads to underachievement that leads to job dissatisfaction that leads to unemployment, divorce, depression (you get the point).

6. Bruce Pennington, Diagnosing Learning Disabilities.

7. Caryl Frankenberger, www.nldline.com