Learning Disabilities as Functions of Familial Learning Problems and Developmental Problems
* In recent years, a consensus concerning the definition of learning disabilities has emerged in the literature (although such agreement does not necessarily prevail in practice, particularly as a definition would affect special education; Furey, 1987; Landers, 1987). This consensus was perhaps best expressed by the National Joint Committee for Learning Disabilities (NJCLD) which defined the term learning disability as follows:
Learning disability is 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, Leigh, McNutt, & Larsen, 1981, p. 336)
Critical elements of this definition are the presumptions that learning disabilities (a) result from central nervous dysfunction and, equally as important, (b) are not secondary manifestations of other primary disorders, most particularly environmental deprivation or emotional disorders.
The definition and its underlying concept are neutral, however, concerning what does cause learning disabilities. Various causes have been suggested--such as genetic defects, intrauterine effects, and laterality. One possible cause that, surprisingly, is omitted from the list of causal candidates is postnatal biological factors. None of these potential contributants to learning disabilities, of course, is exclusive of the others. Other important aspects of learning disabilities, such as their prevalence and incidence, secular changes, and correlates, also remain controversial.
Interest in the possible influence of genetics dates back nearly as far as the first term used to describe learning disabilities and may be implied by the term itself ("congenital word blindness"; Kussmaul, 1877). Empirical evidence supporting the hypothesis that genetic factors had an effect on learning disabilities was first supplied by Thomas (1905). Further empirical support has been provided by family studies (Eustis, 1947a, 1947b; Kagen, cited by Owen, 1977; Norrie, cited by Thompson, 1954; Orton, 1930) and twin studies (Hermann, 1959).
Efforts to identify specific subtypes of learning disabilities may also be traced nearly as far back as the concept itself (Hinshelwood, 1895; Morgan, 1896). Accurate subtyping of learning disabilities is potentially significant to their diagnosis and treatment; but subtyping has a particular bearing on testing the hypothesis that genetic factors contribute to learning disabilities. Particular subtypes of any disorder with distinguishing phenomenological features and specific concomitant markers lend themselves to family studies, medical genetics, and Mendelian analysis.
By contrast, heterogeneous agglomerations of phenomenology are much more difficult to trace through families, let alone through populations. The strategy of combining subtyping with examination of potential genetic determinants has been used to great advantage in the understanding of the etiology of mental retardation, another extremely heterogeneous domain (Childs, Finucci, & Preston, 1977).
Accordingly, strong calls for accurate subtyping of learning disabilities have been issued (Childs et al., 1977), and much recent research designed to test the hypothesis of genetic contributions to learning disabilities has used precisely this strategy. …