Siblings of those diagnosed with autism are more than 20 times as likely as members of the general population to also have autism. Some of these siblings also show evidence of autism-like but less marked cognitive and social communication problems. This suggests that autism has either an environmental cause typically found in all siblings during development or childhood or a strong heritable component, but there is not a known genetic link or a well established biological marker. A biological marker other than observed behavioral deficits would be a neurological phenotype such as might be seen in brain imaging.

Spencer et al, working from various units of Cambridge University seem to have identified such a marker.
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