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That's a good question -- i would ask whether a "high functioning" person is really afflicted with the same physical symptoms as someone with a profoundly disabling autism disorder?

Until recently, these folks would not have been diagnosed with the same disease.

To get a structural MRI scan, you need the subject to both be willing to lie inside a a scanner and capable of holding still for a few minutes.

This can be challenging in populations such as autism, and unfortunately often biases studies towards higher-functioning individuals, who tend to have an easier time with this.

because it muddies the waters. HFA is a fairly defined group but the entire spectrum is a broad category of related disorders. it makes much better sense to do a separate study for other autistic disorders.