The thread title neglects to note that the study focussed on children who had an older, autistic sibling.
The fact that I am the younger of two siblings and an aspie tells us nothing about whether or not my brother is an NT. He is--but the Force (tm) runs strong with him (he has plenty of Aspie traits).
The fact that johnsmcjohn is the eldest of three, with two NT younger siblings simply means that both of his siblings are in the majority--after all, the study only found about 20% of the younger siblings had clinical presentations, and only about 8% warranted an autism disorder diagnosis. Those are high numbers, to be sure, but nowhere close to certainty.
I'm not sure what this really tells us, though. The genetic link has been pretty well documented for some time, but how to we account for environmental factors? How much of our behaviour is determined by the behaviours demonstrated to us by our parents? How much of our communication skill is taught to us by older siblings, where one is available? Does an older, autistic sibling increase the likelihood of genetic predispositions manifesting? These are all viable questions for further research.
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--James