Unsurprisingly, there is plenty of evidence that the chance of a child being born with autism is much higher if they have an older sibling on the spectrum. In fact, as more data has become available, this probability is understood to be even higher than previously thought. In addition, non-autistic siblings often imitate autistic behaviour. In the early 90's when time my brother and I were diagnosed, autism "experts" my parents encountered were often interested in study our genetics make-up. Parents who had an autistic child frequently stopped having more, so we were a rarity.
Over the years as I've met more families with autism, I've noticed that younger siblings often seem to have stronger autistic symptoms- such as more speech and developmental delays than their older autistic brothers and sisters. My younger brother and Dan's were both delayed in speech, whereas were considered quite precocious. Dan went straight from Dr. Seuss to National Geographic while his brother struggled to read. So why is this?
If a child proves more difficult to raise, parents may wait longer to have another child, thus increasing the likelihood of disability in the next. My brother and I are 3 years apart, as are Dan & his brother, and our mothers were both 30 when the had us. So not considered old enough to be risky. Are the parents a little worn out raising the first, so they are a little less conscientious with the second? My dad has joked that its the reverse- that the first child is a sort of "guinea pig" and with the second kid the parents know what they are doing more!
Of course this isn't always the case, I've certainly plenty of autistic kids with younger non-autistic siblings (though frequently they have traits, or related disabilities like AD/HD- and one or both or the parents- often identified after the child with more severe symptoms has been. It's interesting to see the reaction of the parents when another child is labelled (older or younger) Sometimes they are more accepting, in other cases more resistant. (Nooo- why can't I have a "normal" kid!)
I'd be interested in other people's observations and experiences- Please share them below, or post a link to your own blog or website.