Individual variation
Primary school math development is characterized by great individual variation. Most children meet the national math proficiency requirements, whereas some cannot reach the minimum goals of primary math education. When the difficulties in learning mathematics are severe and persistent, this is defined as mathematical learning difficulties (MLD) in the present research project. MLDs have traditionally been viewed as being explicitly different from typical development (TD). However, this way of defining MLD has some severe shortcomings, because a solid empirical basis for the exact boundary between MLD and TD is lacking—also about the cognitive skills associated with learning mathematics. In the literature, it has been hypothesized about subtypes of MLD, but these have not consistently been identified in empirical research either. Therefore, this research project aimed to investigate how MLD subgroups (i.e., subdivided based on different performance profiles, with or without comorbid difficulties, and with more or less stringent diagnostic criteria) differ from each other and from typical development (TD). The central research question was to what extent children with MLD differ from TD children in terms of math performance, development, and associated cognitive skills. Mathematics was divided into basic arithmetic fact retrieval and advanced mathematical problem solving, and the cognitive abilities included number sense, working memory, nonverbal reasoning, and phonological skills. The main conclusion of the present research project is that children with MLD, regardless of how this group is constructed, on average, have similar cognitive profiles to aid math development as TD children. However, substantial individual variation between children exists.