The relationship between infant developmental milestones and later intellectual function was determined in a representative sample of 5,362 children born in the United Kingdom in 1946, after the second world war, and followed in to adulthood by researchers at the Behavioral and Clinical Neuroscience Institute, University of Cambridge; and University College London, UK. When participants reached 2 years of age, mothers were asked about ages of standing and walking, and saying words. Information on at least one cognitive test (IQ at age 8, reading comprehension at age 26, or verbal fluency at age 53) and developmental variables was available in 3,969 subjects. Linear modeling demonstrated that earlier motor and speech development were significantly associated with greater IQ at age 8, higher reading compehension at age 26, and better verbal fluency at age 57. For every month earlier in learning to stand, the individual gained one half of one IQ point at age 8. Earlier development in speech and motor milestones was also associated with better reading comprehension at age 26, but not at age 53. The effect on reading was not solely driven by late developers. Speech development was related to reading comprehension at age 26 even when late developers were excluded. Later speech developers were less likely to progress beyond basic education. 
COMMENT. Later development of motor milestones and speech is associated with a small but significant impairment of intellectual function and educational attainment. The authors postulate a connection between delayed development of cortical-subcortical circuits in infancy and later cognitive dysfunction. A correlation between severe delay in developmental milestones and subsequent learning disability or mental retardation is well documented . In contrast, in the normal population it has been assumed that mild to moderate modifications of motor and speech developmental milestones are not related to later intellectual function. In a recent longitudinal birth cohort study by the Cambridge group of a Finnish population-based sample, it was found that a more rapid infant motor development is linked to better school performance and higher adult IQ scores . Frontal cortex, basal ganglia, and cerebellum volume is linearly related to the speed of infant motor development, according to one recent report . The importance of developmental studies in the prediction of later educational attainment, indicated by the above reports, is also suggested by earlier but smaller nonepidemiological samples cited by the authors. In one report of a primary care cohort of 213 children, the age of learning to walk (a milestone most frequently remembered by a parent) was significantly associated with IQ at age 3 .