*Nearly everyone has heard of developmental dyslexia - a learning disorder characterized by poor reading skills despite otherwise sufficient schooling - but have you heard of developmental dyscalculia? Many people have not. Here is part 2 in a week-long series on this lesser-known learning disorder. (See part one, and a companion post on comparative numerical cognition in humans and animals at The Thoughtful Animal)*

## Developmental Dyscalculia: Definition, Prevalence, and Prognosis

If we're going to seriously discuss a developmental learning disorder, the first thing that might be done is to define it. Ruth Shalev and colleagues, from the Shaare Tzedek Medical Center in Jerusalem, have provided two different definitions for developmental dyscalculia. First, they offer that developmental dyscalculia is a specific, genetically determined learning disability in a child with normal intelligence. The usefulness of this definition, however, is limited when it comes to differentiating students with dyscalculia and students who are simply weak in arithmetic. A more recent definition according to the DSM-IV-R is offered as well, which defines developmental dyscalculia as a **learning disability in mathematics, the diagnosis of which is established when arithmetic performance is substantially below that expected for age, intelligence, and education**.

Prevalence studies have been carried out in various parts of the world, all with (surprise!) different definitions for developmental dyscalculia. Despite the definitional inconsistency, the prevalence of developmental dyscalculia across countries is fairly uniform, at about **3-6% of the school population**. That percentage is similar to the the prevalence of other developmental learning disorders, such as developmental dyslexia and attention deficit/hyperactivity disorder (ADHD).

The manifestation of developmental dyscalculia generally changes with age and grade. First graders (age 5-6) typically present with problems in the retrieval of basic arithmetic facts and in basic computational exercises. By the time children reach age 9-10, they've finally mastered counting skills, are able to match written Arabic numerals to quantities of objects, understand concepts of equivalence or inequivalence, and understand the ordinal value of numbers. They also are generally proficient with handling money and understanding the calendar, two skills which require basic arithmetic proficiency. Instead, children diagnosed with developmental dyscalculia at this age present with deficits in the retrieval of overlearned information such as multiplication tables. In an attempt to bypass their difficulty in solving basic arithmetic problems, these children will use inefficient strategies in calculation. Errors typically include inattention to the mathematical operator, use of the wrong sign, forgetting to “carry over,” or misplacement of digits.

The best kind of study of a developmental learning disorder is one in which the same groups of individuals are studied over the course of months or years, in what is called a longitudinal study. Choose your favorite overused analogy: longitudinal studies are the gold standard, the holy grail, the raison d'être of developmental scientists. Longitudinal studies of dyscalculia, however, are few and far between, so not much is known about the prognosis of those individuals who are diagnosed with developmental dyscalculia. In one short longitudinal study, Shalev and her colleagues examined a group of 140 ten and eleven year old children who had developmental dyscalculia, and reexamined them at age thirteen and fourteen. Their performance, after three years, was still poor, with 95% of the group scoring in the lowest quartile of their school class. Fifty percent continued to meet the research criteria for developmental dyscalculia. The group did a second follow-up in 2005, when the group was finishing high school, at age sixteen and seventeen:

- 51% of the group could not solve 7x8 (versus 17% of controls);
- 71% could not solve 37x24 (versus 27%);
- 49% could not solve 45x3 (versus 15%); and
- 63% could not solve 5/9 + 2/9 (versus 17%).

Forty percent of the group scored in the lowest fifth centile for their grade; ninety-one percent in the lowest quartile. Children whose diagnosis of developmental dyscalculia had persisted also presented with more behavioral and emotional problems than those who no longer qualified for the diagnosis. These problems included anxiety/depression, somatic problems, withdrawal, aggression, and delinquent behavior. Cognitive factors associated with persistent developmental dyscalculia were lower IQ, inattention, and writing problems.

Unlike dyslexia, ADHD, and other learning disorders, which affect more males than females, developmental dyscalculia shows a more equal distribution between the sexes. To date, no convincing answer has been offered for why the usual predominance of boys is not observed for developmental dyscalculia. Many researchers have attributed other non-neurological factors to the etiology of developmental dyscalculia*, some of which may preferentially impact girls more than boys, including lower socioeconomic status, mathematics-induced anxiety, overcrowded classrooms, and more mainstreaming in schools. Differential treatment towards girls by math teachers is also a potential confound.

*This brings up an important point for any psychopathology, which is the notion of **equifinality**. When a pathology (such as dyscalculia, but also e.g. depression, social anxiety, schizophrenia, any of the personality disorders, etc), is defined based on presentation of symptoms, there are often multiple biological and experiential trajectories that can result in such an outcome. One subset of individuals who are diagnosed with dyscalculia may possess some genetic variant that impairs their numerical abilities, while another set of individuals may show the same set of symptoms due to environmental factors such as SES or gender. This is why it is so hard to study psychopathology, and why any one variable only accounts for a small amount of the variance in a disorder.

*Image source.*

**Get Your Literature On**

Shalev, R., Manor, O., Kerem, B., Ayali, M., Badichi, N., Friedlander, Y., & Gross-Tsur, V. (2001). Developmental Dyscalculia Is a Familial Learning Disability Journal of Learning Disabilities, 34 (1), 59-65 DOI: 10.1177/002221940103400105

Shalev, R., & Gross-Tzur, V. (2001). Developmental dyscalculia Pediatric Neurology, 24 (5), 337-342 DOI: 10.1016/S0887-8994(00)00258-7

Shalev, R., Auerbach, J., Manor, O., & Gross-Tsur, V. (2000). Developmental dyscalculia: prevalence and prognosis European Child & Adolescent Psychiatry, 9 (S2) DOI: 10.1007/s007870070009