Background.  The ability to compare numbers, as the most basic form of number sense, has been related to arithmetical achievement. Aims.  The current study addressed the predictive value of non‐symbolic and symbolic (number word (NW) and Arabic number (AN)) comparison for arithmetics by means of a longitudinal design. Sample.  Sixteen children with mathematical disabilities (MD), 64 low achievers (LA), and 315 typical achieving (TA) children were followed from kindergarten till grade 2. Method.  The association of comparison skills with arithmetical skills in grades l and 2 was studied. The performances of MD, LA and TA children were compared. Results.  Regression analyses showed that non‐symbolic skills in kindergarten were predictively related to arithmetical achievement 1 year later and fact retrieval 2 years later. AN comparison was predictively related to procedural calculation 2 years later. In grade 2, there was an association between both symbolic tasks and arithmetical achievement. Children with MD already had deficits in non‐symbolic and symbolic AN comparison in kindergarten, whereas in grade 2 the deficits in processing symbolic information remained. Conclusions.  The combination of non‐symbolic and symbolic deficits represents a risk of developing MD.

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Key messages• Co-morbidity of motor problems with pervasive developmental disorder, a hyperkinetic disorder and/or a speech, language or learning disability varied between one fourth and one third of the children in this clinical sample.• Co-morbidity rates of motor problems in developmental and behavioural disorders are lower than previously reported • Co-morbidity of motor problems with pervasive developmental disorder, a hyperkinetic disorder and/or a speech, language or learning disability shows different patterns in boys and girls • It is recommended to asses motor skills in children with developmental disorders Word count main text: 3167 3 AbstractBackground: Few co-morbidity studies have been conducted since the Leeds Consensus Statement on Developmental Coordination Disorder (DCD) in 2006. In this Statement, international cut-offs and inclusion criteria were agreed and consequently the status of DCD changed. Furthermore, most existing comorbidity studies are small clinical studies, rather than epidemiological studies, resulting in a broad range of co-morbidity rates. DCD has a higher incidence for boys in comparison with girls, questions arise if this preponderance remains the same in combination with other developmental disorders. Therefore, in this study we aimed to determine co-morbidity and gender differences of motor problems in children with a pervasive developmental disorder, a hyperkinetic disorder and/or a speech, language or learning disability. Methods: Profiles of 3608 children (mean age 9y 1mo) referred to rehabilitation centres for behavioural, developmental and sensorineural disorders were studied.Results: Motor problems were reported in one fifth of the total sample. Comorbidity of motor problems in specific disorders varied between almost one fourth to more than one third. The male/female ratio was significantly higher in children with motor problems and 2 or more other disorders compared to children with motor problems and less than 2 other disorders.Conclusions: This study indicates that co-morbidity of motor problems with other clinical disorders is not exceptional and developmental deviance is seldom specific to one domain. However, current co-morbidity studies tend to overestimate the number of children with motor problems. In addition, there may be different patterns of symptoms between the genders. These findings stress the importance of assessing motor skills in children with various developmental disorders.

A relationship between motor and mathematical skills has been shown by previous research. However, the question of whether subtypes can be differentiated within developmental coordination disorder (DCD) and/or mathematical learning disability (MLD) remains unresolved. In a sample of children with and without DCD and/or MLD, a data-driven model-based clustering was used to identify subgroups of individuals with relatively homogeneous profiles on measures associated with motor and mathematical skills. One subgroup of children with motor problems was found based on motor variables. Based on mathematical variables, two clinical clusters were found: a subtype with number fact retrieval problems and a subtype with procedural calculation problems. Clustering with motor and mathematical skills revealed two clinical clusters: a cluster with number fact retrieval as well as procedural calculation problems and below average motor and visual-motor integration skills. A second cluster of children had only procedural calculation and visual-motor problems. Our results raise questions about the usefulness of placing children who have below average mathematical skills into a single diagnostic category. Furthermore, we inform ongoing debates about the overlap between DCD and MLD, as below average motor skills were found in both MLD subgroups, although a different motor profile is linked to a different mathematical profile.