New University of Otago research into two sex hormones released by the testes of male fetuses and boys may help solve the enduring mystery of why autism is much more common in boys than girls.

Department of Anatomy researchers have discovered that variations within normal- range levels of anti-Müllerian hormone (AMH, also known as Müllerian inhibiting substance) and inhibin B (InhB) are linked with the severity of symptoms in boys with autistic spectrum disorders (ASD).

Autism spectrum disorders are developmental disorders featuring repetitive or stereotyped behaviours as well as impairments to social interaction, communication and language.

The new Otago study, which challenges current thinking that ASD simply reflects a testosterone-fuelled extreme of male biology, was carried out by Dr Michael Pankhurst and Professor Ian McLennan and is newly published in the international journal Translational Psychiatry.

The researchers studied blood samples from 82 boys with ASD and 16 control boys, all aged between 4.4 to 8.9 years. Measuring the levels of the two hormones, the researchers found that these were highly variable from boy to boy, but no different on average between the two groups of boys.

However, in boys with ASD, those with high InhB levels tended to have worse autistic symptoms than those with low levels of the hormone. Conversely, ASD boys with high AMH levels tended to have fewer symptoms.

Professor McLennan says the findings indicate that male hormones are important for autism, but not because autistic boys have abnormal levels.

“While it has been previously suggested that exposure in the womb to excessive levels of testosterone might be creating an ‘extreme male brain’, this does not explain why some females have autism, or why males with autism do not exhibit an extreme male physical form.