Motor development in infancy is directly related to cognitive function at 4 years of age. This relationship is most pronounced for infant motor variation and performance. The size of the differences in IQ is clinically relevant.

The study was financially supported by the University Medical Center Groningen, grant number: 754510, the Junior Scientific Masterclass, the Postgraduate School Behavioural and Cognitive Neurosciences and the Cornelia Foundation, Groningen, The Netherlands. The authors have no conflicts of interest to declare.

The authors have no conflicts of interest to declare. The study was supported by the University Medical Center Groningen, the Cornelia Foundation and the school for Behavioral- and Cognitive Neurosciences. The sponsors of the study had no role in study design, data collection, data analysis, data interpretation or writing of the report.