Differences in sex ratios in autism as a function of measured intelligence.

The team looked at old IQ files for kids already in clinics. They counted how many boys and girls had autism at each IQ level.

They did the same count for kids with other diagnoses to see if the pattern was special to autism.

Girls with autism showed up most often in the very lowest IQ band. Boys with autism were spread across high and low IQs.

Kids with other diagnoses did not show this girl-heavy low-IQ pattern.

Catania et al. (1982) saw the same thing two years earlier: autistic boys scored higher on non-verbal tests. The 1985 paper turned that score gap into a head-count gap.

Banach et al. (2009) split families into simplex and multiplex. The low-IQ girl pattern only held in simplex families. This tells us the 1985 finding is not true for every group.

Panahi et al. (2023) found boys, not girls, had shorter telomeres. Each study shows a different sex marker, but all point to biology working differently in autistic boys and girls.

If you test a girl with very low IQ and social delay, think autism first. Do not wait for classic repetitive play; she may not show it. Use the same full test battery you give boys, and track family type if you can. Early, correct diagnosis gives her faster access to language and daily-living interventions.