Objective: Developmental delay is associated with a subsequent diagnosis of learning disability. However, the relationship between the age of reaching infant developmental milestones and later intellectual function within the general population remains unresolved. We hypothesized that earlier attainment of developmental milestones would be associated with better subsequent intellectual performance throughout the range of abilities, rather than confined to extremes.
Methods: Developmental data were obtained at age 2 years in the National Survey of Health and Development, a representative sample of 5,362 children born in the United Kingdom in 1946. Data on intellectual function and educational attainment at ages 8, 26, and 53 years were also obtained. Multiple linear regression and logistic regression were used to analyze the effect of age of reaching developmental milestones on subsequent cognition and educational attainment.
Results: The age of reaching developmental milestones was associated with intellectual performance at ages 8, 26, and 53 years; for every month earlier a child learned to stand, there was, on average, a gain of one half of one intelligence quotient point at age 8. Speech development had a small but statistically significant effect on subsequent educational attainment (later developers were less likely to progress beyond basic education); this effect was not apparent for motor development. Effect sizes were reduced when the slowest developers were excluded, but many effects remained significant.
Interpretation: The association between later development and poorer subsequent intellectual function is small, but it does have theoretical implications; we suggest it is secondary to suboptimal cortical-subcortical connectivity.