When is impairment in development of motor coordination not solely explicable in terms of intelligence?

Objective: In both clinical practice and research, motor delay is understood to be explained, at least in part, by intellectual abilities. However, no data are available in order to operationalise these criteria to guide clinical decision making. This study provides data on IQ and motor skill in children to answer three research questions concerning the relationship between IQ and motor skill: (1) Can motor coordination impairment be explained in terms of general intellectual retardation? (2) What level of motor performance is to be expected given the person's measured intelligence? (3) At what point are motor difficulties considered to be in excess of those usually associated with mental retardation? Participants & Methods: IQ and motor skill data were analysed from a group of 460 children identified with/without motor difficulties from both clinical and educational settings. Results: Typical and atypical motor skill was seen at all IQ levels, 19% of the variance in motor outcomes was explained by IQ scores, and for each standard deviation lower IQ, a mean loss of 10 percentile motor points should be expected. Conclusions: Although individuals with a lower measured IQ more often showed poorer motor performance than those with a higher measured IQ, motor skill at all levels of proficiency was seen in all IQ categories. These findings have important implications for clinical judgements and decision-making as well as for future research directions to further operationalise the criteria relating to motor disorders in both DSM-IV and ICD-10