Gender Differences in Autistic Spectrum Disorders

Background: Autism spectrum disorders (ASDs) are more common in males than females. The gender-ratio increases (up to 10:1 or more) with greater intellectual ability. Males may be generally more vulnerable to genetic risk of ASD. Alternatively, our current conceptualization of autism could be biased toward recognising the male phenotype, leading to underdiagnosis in females. To date, attempts to differentiate the female ASD phenotype have yielded inconsistent results, which may be due to the use of small samples and an excess of intellectually low-functioning participants.

Objectives:

We aimed to test, in a large clinically ascertained sample with normal range intelligence, the consistency of the phenotypic profile of males and females diagnosed with autistic spectrum disorders. Our main objective was to derive precise quantitative measures of impairment. To this end we identified similarities and differences, by gender, using detailed phenotyping based upon a range of standardized measures of behaviour.

Methods:

Data from parental report, questionnaires and observational measures were analysed for a total sample of 493 boys and 100 girls. A smaller age and IQ matched sample of cases (25 boys and 25 girls) was examined in greater detail, to identify more subtle characteristics, in terms of both qualitative and quantitative features, of conventionally ascribed autistic behaviour.

Results:

There were no differences in terms of social or language deficits, in terms of quantity or quality, between the sexes. Girls showed significantly fewer stereotyped and repetitive behaviours than boys and, when present, these behaviours differed significantly from the male pattern in several qualitative characteristics. There were no group differences in general intellectual ability. However,  a history of epilepsy and disorders of affect was signficantly more common amongst females with ASD.

Conclusions:

Gender differences in the severity and symptomatic content of repetitive and stereotyped behaviours may lead to a bias against the identification of high-functioning girls with ASD.