Brain and Mind: The Relationship Between Head Circumference Trajectories and Intelligence in Typically Developing Infants

Background:  Abnormal brain growth trajectories are increasingly recognised as a key developmental feature of autism spectrum disorders (i.e. Courchesne, Campbell, & Solso, 2011), although little is understood about the cognitive implications of these growth patterns. In typically developing individuals, it is recognised that individual differences in brain size and growth trajectories during infancy may predict later IQ performance (i.e. Gale et al., 2004;2006), but no studies have investigated the concurrent covariance between brain growth and cognitive growth despite the implications of such a potential coupling for both clinical and typical populations.  

Objectives:  This study therefore aimed to explore the relationship between brain growth, as indexed by head circumference (HC), and IQ performance in a longitudinal sample of typically developing infants 7-29 months old. HC is a very strong predictor of brain volume in young children (Hazlett et al., 2005). 

Methods:  A total of 25 infants were seen on 3 occasions where IQ was assessed using the Mullen Scales of Early Learning (Mullen, 1995) and HC was measured by a trained researcher using a non-stretchable fibreglass measuring tape. Mixed linear growth models with random intercepts and slopes, were conducted to investigate the relationship between IQ change and HC growth. 

Results:  Between-child IQ performance and within-child changes in IQ performance showed distinct relationships with HC size and growth. Higher IQ scores at the first session (between-child IQ) were associated with larger HC as well as slower and less decelerative HC growth rates, lower IQ with faster and more decelerative HC growth and a smaller HC. Within-child changes in IQ performance at the second or third visit, on the other hand, were associated with a contemporaneous change in HC growth rates, IQ increases associated with faster growth rates.  

Conclusions:  This is the first study to report that individual differences in IQ performance are associated with different HC growth rates in infancy at both the population and the individual level, and furthermore that between-child IQ scores (how they compared to other children in the sample) showed a different relationship with HC than did within-child IQ score changes (how much each child varied in IQ performance across time). Implications are discussed in terms of possible biological mechanisms, the effects of abnormal brain growth in autism on cognitive development, and the use of IQ or HC measurements in infant research studies.