A) Genetics
B) becuase in many cases syndromes such as ASD, ADHD etc are not picked up until later. And the stats aren;t known or certain either- so if you have one child with asd there's currently no way to tell whether it's genetic or compeltely unrelated. And if it is genetic the odds are still officially below 15% of a recurrence.
in our case certainly when ds3 was born we had no idea of the ASD genetics, and when ds4 was born all the boys were in mainstream education and getting along: and becuase ds2 appeared NT that we were told the odds were also in our favour. Also as I was an at home carer anyway and we didn't have to rely on state support it didn;t seem such a big risk: ds3 being teh exception who was too young to identify at that stage, the boys should all be economically active as adults. DS1 has a career that is well within his reach all planned out.
Will I have any more given the risks now known? no. But the boys were 5 and 6 when diagnosed, ds4 aged 3 when we cottoned on and ds2 a whole ten years old.
Some disabilities reveal themselves through tests or at birth, a great many do not. I was absolutely sure ds3 was NT for years but he lost skills. Went from berbal to not in a few days. He is the one I get carer's allowance for (it's only given once per family, probably rightly).
DS2, the most likely to have children of his own, is well aware of the increased risks he now ahs but it's a science still in it's infancy: hopefully by the time he is adult we will know far more about ASD and genetics. After all, my field is ASD and you still get all manner of theories through- very rarely is it accepted as a genetic disease except in famillies like mine. gradually risks and triggers are being identified and we might know an awful lot more soon. It's only 20 years since AS was really shown to be distinct in the UK from generic learning disabilities, we have such a very long way to go. And whilst DS1 has an exceptional talent he can pursue, for most people with AS (that's the level of 3 of my 4- there was no reason to suspect ds3 would be more functionally severe and indeed there was a head injury that may have been involved ) the most important thing to be learned is how to enable them to access ecducation and work effectively and live to their potential: hopefully my MA in ASD will help me manage that for the boys, I wish I could see more work in that line going on elsewhere. Far better investing money in maximising independence and life chances than maintaining people on benefits.
Actually I think that's where teh system is most screwed: the first things cut were the tings that helped people off benefits. They shouod be the last things to go. From relate counselling to proper differentiated education, they pay for themselves in the long term.