What is the difference between reading disability and arithmetic disability?

According to 4-DSM (American Psychiatric Diagnostic Book), Dyscalculia is a specific learning disability in arithmetic, and refers to low achievement in arithmetic in relation to what is expected according to the person’s chronological age, intelligence or education (American Psychiatric Association, 2000). The low achievements interfere with the functioning or academic achievements and do not result from difficulties in one of the senses. The prevalence of this disorder is 6% of the population, and is expressed in a number of areas: counting, arithmetic strategies, recalling of arithmetic facts, location of numbers on an axis, comparison of sizes and estimated calculation (Wilson et al., 2015).

Dyslexia is defined in a similar way to dyscalculia except that the difficulties are in reading and not in arithmetic and it is manifested in slow and/or distorted reading. There is strong evidence that dyslexia is a largely inherited disorder, with a prevalence of 5-11% (Wilson et al., 2015).

The 4-DSM is based on the single deficit machine model, according to which all cases of a specific disorder result from the same single factor in a ratio of 1: 1. i.e. while all cases of dyslexia are caused by a cause X, all cases of dyscalculia are caused by a cause Y (Willcutt et al., 2010).

A significant criticism of this model is that it does not explain the high overlap between the various disorders. For example, if dyscalculia and dyslexia were indeed independent disorders of various causes, we would expect the percentage of people with dyslexia to be similar among both people with dyscalculia and people without dyscalculia (about 5-11%). In practice, this is not the case and there is a high comorbidity between the disorders; it was found that 25.9% of people with significant difficulties in mathematics also have significant difficulties in reading – much more than is expected in the general population (Landerl et al., 2013).

A more up-to-date model is called multiple-deficit and refers to disorders as complex conditions caused by a number of genetic and environmental risk factors, some of which affect each of the disorders separately and some of them jointly (Willcutt et al., 2010). The 5-DSM compatible with this model. According to which, all learning disabilities (dyslexia, dyscalculia and written expression) should fall under the category of “specific learning disabilities”. In other words, due to the many comorbidities that exist between the disabilities, it can be said that they are all considered to have one learning disability when only the emphasis of the difficulties is different in each individual (American Psychiatric Association, 2013).

Several studies have been conducted to examine the causes of comorbidities between arithmetic disabilities and reading disabilities. For example, a study of twins from Denver in the United States found that there was a significant genetic factor which is common to reading difficulties and arithmetic difficulties (Light & DeFries, 1995). Another study, which focused on the cognitive factors for disabilities, showed that verbal abilities are a common factor in both arithmetic and reading abilities and can therefore explain the many comorbidities between the disabilities.

In addition, the study found that phonological awareness (ability to distinguish sounds within a word) and naming speed contribute uniquely to reading capabilities  and on the other hand visual working memory contributes uniquely to arithmetic capabilities  (Peterson et al., 2016).

In summary, according to the theory of multiple-deficit, it was found that there are a number of cognitive impairments some of which contribute uniquely to each of the abilities and some of which are common to both arithmetic and reading abilities.