I assume this is the McGuffin's study:
Quote:
http://www.sciencemag.org/cgi/content/full/291/5507/1232
There is a figure in there which is, I believe, the source of the '90 %'. But the reference numbered '11' is not to be found easily as it is a book as I see. I am pretty sure though, since they talk specifically about twin studies, that they meant the risk for a twin to have the disorder if one of the other twins has it already. For identical twins, risk of 90 % is high-ish but not unheard of. For most hereditary and multifactorial disesase the risk for one twin if the other has the disorder varies between 30 and 70 %. For fraternal twins the risk is as it would be between normal siblings (provided that the condition is not caused by some influence both twins experienced when they were in the maternal womb - and even this is not 100 % sure). If the risk for both identical twins to be affected is high, this indicates for a definitive genetic component, while if the risk is higher for ordinary siblings (like fraternal twins) it is more likely to have a shared environmental component, as normal same-parents siblings have only 25 % chance of sharing roughly the same genes.
I won't put my head on the block to defend my thesis but I am pretty sure that by twin studies they meant studies that assessed the risk of both twins having the same disease. There's a link here:
Quote:
http://www.theautismnews.com/2009/10/24/high-risk-of-identical-twin-developing-autism/
which is about 88 % (very close to those 90%) risk of identical twins having autism provided that one of them is autistic. I believe this is the case.