Head circumferences in twins with and without Autism Spectrum Disorders.

Froehlich et al. (2013) measured head size in twin pairs. One twin had autism, the other did not.

They used a tape measure around the widest part of the skull. They noted how many kids had macrocephaly, that is, a head size above the 97th percentile.

Head size was the same in both groups. The autistic twins were not more likely to have big heads.

About the same number of unaffected twins also had macrocephaly. The trait ran in families, not in autism itself.

McMillan et al. (1999) saw macrocephaly in 1 of every 6 autistic children. Wendy et al. show the rate is the same in unaffected co-twins, so the big head is inherited, not caused by autism.

Cederlund et al. (2014) found only 3 % macrocephaly in preschoolers with autism. The low number looks like a clash, but Mats studied random preschoolers while Wendy studied twins. Twins share genes, so the inherited big-head trait shows up more clearly in them.

Geurts et al. (2008) reported larger head size at 12 months in infant siblings who later developed autism. Again, the groups differ: infant siblings are high-risk babies, while Wendy’s twins were school-age. Early fast growth may fade by middle childhood.

Stop using macrocephaly as an autism red flag. Instead, see it as a family trait that may sit next to autism but does not cause it. When you measure head circumference, compare it with the child’s siblings and parents, not just growth charts. This shift keeps you from over-interpreting a single physical sign.