Two of the three studies on autism were American-led and their findings are reported in the online journal Nature.

The third study was conducted by British scientists at Oxford University and appears in the journal Molecular Psychiatry.

The research involved screening the DNA of many thousands of volunteers.

The largest investigation, conducted in the US, screened almost 13,000 people, including many from families affected by autism.

Comparing the entire genetic code of these individuals enabled scientists to focus on a “hotspot” on chromosome five – one of the coiled up “packets” of DNA within the nuclei of cells that contain the genes.

“Although a particular gene variant may contribute a small risk for an autism spectrum disorder (ASD) in a particular individual, we estimate that the variants we discovered may contribute to as many as 15 per cent of ASD cases in a population typically referred to as the population-attributed risk of the variant,” said study leader Dr Hakon Hakonarson, director of the Centre for Applied Genomics at The Children’s Hospital of Philadelphia.

The British scientists, who analysed the DNA of more than 500 people, looked for single-letter changes in the genetic code in two particular chromosomes as well as copy number variations.

They pinpointed a gene on chromosome 7 called Dock4 that appeared to be significantly associated with autism.

Tara Matthews, spokeswoman for the Irish Society for Autism, said the research was another step forward in trying to find an answer to what caused autism.

Professor Michael Fitzgerald pointed out that “autism is a very complex disorder both in description and genetics . . . there are many, many genes involved . . . we might be able to be more hopeful about certain subtypes where we might be able to make more progress from a therapeutic point of view because of this [research]". (Source: Irish Times)