Associations of Early Developmental Milestones With Adult Intelligence

Abstract

The study investigated whether age at attainment of 20 developmental milestones within the areas of language, walking, eating, dressing, social interaction, and toilet training was associated with adult intelligence. Mothers of 821 children of the Copenhagen Perinatal Cohort recorded 20 developmental milestones at a 3-year examination, and all children were administered the Wechsler Adult Intelligence Scale when they were 20-34 years old. Later attainment of a number of milestones was associated with lower adult IQ with the strongest associations found for those related to language and social interaction. The adjusted full-scale IQ means were 107.0, 101.8, and 100.6 for being able to form a sentence at less than 24 months, at 24 months, and later than 24 months.

Language development and intelligence in midlife

“…Three previous longitudinal studies are based on a study design in which cognitive outcomes are measured in adulthood or adolescence: Two of these studies are based on the same birth cohort as the present study, and one study used the BPP as outcome completed at a mean follow‐up age of 19.3 years (Flensborg‐Madsen & Mortensen, ), while the other used WAIS administered at a mean adult age of 27.6 years (Flensborg‐Madsen & Mortensen, ). The studies investigated the timing of 20 developmental milestones assessed during the age from one to three years within the areas of language, walking, eating, dressing, social interaction, and toilet training, and associations were, also in these studies, analysed by multiple linear regressions adjusting for potential confounding factors.…”

“…Although two similar subtests are included in both the BPP and the I‐S‐T 2000 R, these tests are not directly comparable, and the WAIS is a much more comprehensive measure of intelligence than the other test. However, comparison of the standardized regressions coefficients suggests that the coefficients for midlife intelligence may be at least as high as those for the BPP (Flensborg‐Madsen & Mortensen, ), while the coefficients for the WAIS may be slightly higher (Flensborg‐Madsen & Mortensen, ). Thus, it may be cautiously concluded that the present study provides no evidence that the influence of early language development is diluted over the adult life course.…”

“…At a 3‐year CPC follow‐up assessment, the mothers were interviewed and asked to retrospectively recall the child's age at attaining 20 developmental milestones. The milestones covered a wide range of behaviours, but milestones related to language and social interaction were chosen to be the main focus of the present study because they have shown the most robust associations with intelligence in young adulthood (Flensborg‐Madsen & Mortensen, , ) and because milestones related to the other areas were not significantly associated with midlife IQ (Table S2). Table includes a description of the included milestones and presents median age at attainment and the 25% and 75% percentiles of each milestone.…”

“…In two previous studies based on the Copenhagen Perinatal Cohort (CPC), the relationship between the timing of early milestones and intelligence in young adulthood was addressed by examining associations between 20 developmental milestones (including language milestones) attained within the ages of one to three years and intelligence assessed by the Børge Prien's Prøve (BPP) (Teasdale, ) and the Wechsler Adult Intelligence Scale (WAIS) (Wechsler, ) in adulthood at mean ages of 19.3 and 27.6 years, respectively (Flensborg‐Madsen & Mortensen, , ). Several significant associations between the timing of milestones and intelligence were discovered, with the largest associations generally found for milestones related to language and social interaction; the latter primarily including milestones reflecting social aspects of language and communication.…”

Early life predictors of intelligence in young adulthood and middle age

“…Weight, length, and head increase from ages 1-3 years were calculated based on information from the 1-year and the 3-year followup. The 3-year milestones mean was calculated as the mean age of attaining 20 milestones within the areas of language, walking, eating, dressing, social interaction, and toilet training achieved between ages 1-3 years and retrospectively recalled by the mother at a 3-year examination; detailed descriptions of this data collection and the milestones can be found elsewhere [32].…”