Head circumference and intelligence, schooling, employment, and income: a systematic review.

Background: No consensus exists about the role of head circumference in identifying children at risk of suboptimal development. The objective of this study was to evaluate the association between head circumference and intelligence, schooling, employment, and income. The review 1) summarizes the overall evidence and 2) restricts the evidence to a subset of articles that met minimum quality criteria. Methods: PubMed, Web of Science, PsycINFO, LILACS, CINAHL, WHO Institutional Repository for Information Sharing and UNICEF Innocenti were searched to identify published studies. Cohort, case–control or cross-sectional studies which evaluated the associations of interest in the general population, premature babies, babies with low birth weight or small for gestational age were included; head circumference must have been measured before the age of 20 years. Two reviewers independently performed study selection, data extraction and quality assessments. Results: Of 2521 records identified, 115 were included and 21 met the minimum quality criteria. Ninety studies investigated if early measures of head circumference predict later outcomes and 25 studies measured head circumference and the outcome at the same timepoint; 78 studies adjusted the head circumference for age and sex. We identified large heterogeneity and inconsistency in the effect measures and data reported across studies. Despite the relatively large number of included articles, more than 80% presented serious limitations such as lack of adjustment for confounding and severe selection bias. Considering the subset of articles which met the minimum quality criteria, 12 of 16 articles showed positive association between head circumference and intelligence in the general population. However, in premature babies, 2 of 3 articles showed no clear effect. Head circumference was positively associated with academic performance in all investigated samples (5 of 5 articles). No article which evaluated educational attainment and employment met the minimum quality criteria, but the association between head circumference and these outcomes seems to be positive. Conclusions: Larger head circumferences are positively associated with higher levels of intelligence and academic performance in the general population, but there is evidence of non-linearity in those associations. Identifying a group of children in higher risk for worse outcomes by a simple and inexpensive tool could provide an opportunity to mitigate these negative effects. Further research is needed for a deeper understanding of the whole distribution of head circumference and its effect in premature babies. Authors should consider the non-linearity of the association in the data analysis. Trial registration: Association between head circumference and intelligence, educational attainment, employment, and income: A systematic review, CRD42021289998. [ABSTRACT FROM AUTHOR]