Perímetro cefálico e capital humano: coorte de nascimentos de 1993 de Pelotas

Abstract

The role of intelligence throughout the life course is well established, with higher intelligence quotient (IQ) being associated with academic and employment success, higher income in later life, and lower morbidity and mortality rates. Previous studies have shown that head circumference can serve as a reasonable surrogate for brain volume, which may explain its association with intelligence. This is particularly relevant because head circumference is a simple, cost-effective, and easily measurable tool, especially in lowresource settings where both undernutrition and suboptimal development are more prevalent. Given this, this thesis had four primary objectives: 1) To evaluate the association between head circumference and intelligence, schooling, employment, and income in the existing literature; 2) To investigate the association between head circumference at birth, IQ, and educational attainment in young adulthood, exploring the nonlinearity of this relationship; 3) To describe children's head circumference at birth and its growth during the first year of life and 4) To examine the relationship between head circumference growth during the first year of life and IQ at 18 years of age, aiming to identify a critical period in infancy when interventions could be most effective. To achieve these objectives, we conducted three studies: a systematic review with searches on PubMed, Web of Science, PsycINFO, LILACS, CINAHL, the WHO Institutional Repository for Information Sharing, and UNICEF Innocenti, as well as two original studies using data from a population-based birth cohort launched in 1993 in Pelotas, Brazil. We identified significant heterogeneity and inconsistency in effect measures and reported data across studies. Although we included a relatively large number of articles (N=115), over 80% had significant limitations, including inadequate confounder adjustment and severe selection bias. Larger head circumferences were generally associated with increased intelligence and academic performance, but there was evidence of non-linearity in these associations. The relevance of head circumference measurements appears to diminish after two years of age. Although evidence is limited, larger head circumferences also appear to be linked to better educational attainment and employment. However, no study investigated the association between head circumference and income. In our original studies, head circumference at birth was positively associated with both IQ at 18 years and educational attainment at 22 years, with stronger associations observed at the tail of the head circumference distribution. Regardless of head circumference at birth, a one standard deviation (SD) increase in relative head circumference growth during the first year of life was associated with a 1.3-point increase in IQ in young adulthood (95% CI: 0.5, 2.1). This effect was primarily driven by relative growth in the first six months of life, whereas the effect of relative growth from six to twelve months appeared smaller or negligible. Our findings offer an encouraging perspective for individuals who faced adversities during pregnancy. Although the effect of head circumference at birth on later intelligence persists into adulthood, children born with smaller head circumferences can overcome this disadvantage. Importantly, significant catch-up growth was observed in practice, with increases of 2, 3, and even 4 SD during the first year of life. Further research is needed to identify factors influencing head growth in the first six months of life, which could help inform early childhood interventions and optimize cognitive development.