The Costs and Benefits of a Larger Brain: The Relationship Between Head Circumference and Autism-Related Traits In Typically Developing Children 12-65 Months Old

Background:  Differences in childhood head circumference (HC), especially in growth patterns and size, are one of the few consistent findings in the study of autism spectrum disorders.  While individual variations in HC have robustly been found to correlate positively with measures of intelligence in typically developing children, little work has investigated the relationship between HC and autistic-like traits known to occur at sub-clinical levels in the general population.

Objectives: This study utilised typically developing children with no history of autism in the family  to explore whether HC size relates to individual differences in two types of behaviours related to the autism phenotype, sensory processing dysfunction and restrictive/repetitive behaviours.

Methods:  HC was measured either by a researcher or by a parent in 68 children aged 12-65 months and computed into z-scores after accounting for gender and age using the LMS method (Cole & Green, 1992). Sensory processing was assessed in the 30-65 month old children using the Short Sensory Profile (SSP; McIntosh, Miller, & Shyu, 1999; n=41), and repetitive behaviours in the 12-65 month old children by the Repetitive Behaviours Questionnaire (RBQ; Leekam et al., 2007; n=63). A measure of intelligence, the Mullen Scales of Early Learning (Mullen, 1995) was also administered to a subset (n=47) of these children. 

Results:  Correlation analysis revealed that larger HC was significantly associated with higher IQ scores (r=.403), greater total sensory processing problems (r=-.530) and total repetitive behaviours (r=.339), specifically on the insistence on sameness (IS) scale but not the repetitive and sensorimotor (RSM) scale. Controlling for IQ did not change the pattern of correlations between HC and autistic-like traits. Moreover the two questionnaires were strongly correlated with each other and IQ correlated negatively with the RSM scale of the RBQ (r=-.335) but not the IS scale. 

Conclusions:  These results are discussed with regards to the spectral nature of autistic-like traits in the general population and what this implies for autism, as well as a consideration of the implications for the study of normal brain development and intelligence.