Brief report: cognitive correlates of enlarged head circumference in children with autism.

Jennett et al. (2003) measured the heads and IQs of the children with autism. Ages ranged from 3 to 11. They used a cloth tape to get head circumference and gave each child a short IQ test.

The team then asked: do kids with big heads show a special IQ pattern?

One in five autistic children had macrocephaly, defined as a head size above the 97th percentile. That rate is six times higher than in the general population.

These big-headed kids scored high on nonverbal tasks but low on verbal ones. The split was large enough to stand out in the data.

Cederlund et al. (2014) seems to disagree. They found only a large share macrocephaly in preschoolers with ASD, matching typical rates. The gap is age: the 2014 study looked only at toddlers, while K et al. sampled up to age 11. Big heads may emerge after the preschool years.

Pan et al. (2021) pooled 18 studies and confirmed that macrocephaly is more common in autism. Their meta-analysis includes the 2003 finding, showing the trend holds across decades and countries.

Geurts et al. (2008) and Balaum et al. (2026) extend the idea backward to infancy. They show that extreme head growth curves in the first year predict later ASD symptoms. Together, the papers sketch a timeline: unusual head growth can start in infancy, show macrocephaly by mid-childhood, and link to a high-nonverbal, low-verbal IQ profile.

Add head circumference to every autism assessment. A quick tape measure can flag kids who may need deeper cognitive testing. When you see a big head, probe for visuospatial strengths and language delays; the split may guide goal setting and help explain uneven skills to parents.