By Rebecca Morelle
Science reporter, BBC News, York
High levels of a male sex hormone in foetuses are linked to a higher chance of developing autistic traits in childhood, scientists say.
It is still unclear whether testosterone is causing the traits
The findings come from an eight-year study relating the development of 253 children to levels of testosterone they were exposed to in the womb.
The scientists said it was unclear whether the hormone was causing the traits or was a by-product of them.
The research was presented at the BA Festival of Science in York.
The research team, from the University of Cambridge, looked at levels of foetal testosterone in the womb by examining samples taken from women undergoing amniocentesis for clinical reasons.
The children were then followed during their development.
At 12 months, 18 months and 46 months, the scientists used tests to spot autism-like traits, such as counting how often a child looked at its mother's face or how large its vocabulary was.
At these early stages, the team found a link between the traits and higher foetal testosterone levels.
But the scientists' latest research results came from a study undertaken when the children were eight years old.
The children's mothers filled in a questionnaire called the autism spectrum quotient. This is designed to test the number of autistic traits a child has by examining factors such as social interest and pattern recognition.
Typical questions included whether the child preferred social activities like parties or spending time alone, or whether he or she was quick at picking up numerical patterns, like remembering number plates or phone numbers.
The results were then compared with the pre-natal testosterone levels, which had a 20-fold variation, between 0.1 to 2.05 nanomoles per litre.
Bonnie Auyeng, who carried out the study, said: "The correlation is not perfect, but foetal testosterone will account for about 20% of the variability in [questionnaire] scores. Although this doesn't sound like a very high number, it is statistically significant."
Extreme male brain
Professor Simon Baren-Cohen, who was also involved in the study, said: "This is the first time autistic traits have been linked to levels of foetal testosterone, measured in the womb using amniocenteses."
Animal research has previously linked brain development to foetal testosterone levels, and some believe the hormone may play a causal role in autism.
However, the scientists stressed that the study only showed a link between autistic traits and the hormone, rather than a direct link to autism itself.
Dr Auyeng said: "We're still in the early stages of figuring out what actual role foetal testosterone plays. We don't know if it is causing autistic traits, if it is a by-product of them, or an indication of various interactions.
"We are just not sure yet."
Scientists currently do not know what causes elevated levels of foetal testosterone. Professor Baron-Cohen said previous research suggested that it could be a mixture of genetic and environmental factors.
He said that the hormone could be affecting the brain through altering neural cell connectivity and chemicals that carry messages, known as neurotransmitters.
The team is now planning to follow up its study to test direct links between autism and testosterone levels in foetuses. The group will use Denmark's archive of 90,000 amniocentesis samples and its register of psychiatric diagnoses.
The work is connected to Professor Baron-Cohen's hypothesis suggesting that autism is a version of the extreme male brain.
He said that although researchers had tested this theory at the psychological level, the new studies meant it could now be tested at the biological level.