What causes autism? What we know, don’t know and suspect

Another research technique has been to dissect the brains of individuals with autism who have prematurely died, so-called post-mortem studies. A recent study that examined the brains of 11 autistic individuals at the microscopic level found changes in the structure and organisation of the brain cells that form during fetal life, indicating differences in brain development that begin very soon after conception.

Another well-studied area in autism is head circumference growth in the first years of life. This research dates back to 1943 and Leo Kanner’s original study that found five of the 11 children with autism he examined had large heads.

Several small studies throughout the 1990s and 2000s searched the medical records of relatively small groups of children with autism. These found that a key period was the first two years of life, in which a minority of children later diagnosed with autism had a marked increase in the rate of growth of their head.

During the first two years of life, the size of an infant’s head is a reasonable indicator of total brain size, and for many years “brain overgrowth” during very early development was seen as a risk factor for a later diagnosis of autism.


However, more recently, this view has been challenged by the release of the largest ever study in this area, which found no link between infant head circumference growth and autism.

Head size has nothing to do with autism.

Studies using brain imaging machines have examined whether parts of the brains of individuals with autism may be different in size, shape or function.

However, the only consistent finding is just how much inconsistency there is. Not every individual with autism has differences in the size or pattern of growth of different brain regions. For those individuals who do, it is unclear how this may relate to their autistic behaviours.

A great deal of brain imaging research has examined the connections within the brain of individuals with autism. Connectivity is a measure of how well and how much two brain areas communicate with each other. In the study of autism, scientists distinguish between short-range connections (between neighbouring brain areas) and long-range connections (between brain areas further apart).



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