Both. With a dash of their own choices tossed into the mix.
I've never seen a family so riddled with autistic spectrum as husband's family is. I mean......honestly, it is so much so someone might want to actually "study" them if they'd cooperate. Because so far, I've yet to find a single male without it. Although I can only go back as far as husband's Dad and Uncle from personal experience....and have to go by information provided by mother in law as far as husband's grandfather ect.
They are with varying degrees of functioning as well. From severe to very high functioning. I'd classify my father in law and brother in law as very high functioning, as well as husband's cousin (although I think he's only high functioning in that he found a wife that "understands" and the perfect job for him) husband is in the middle. Another cousin is rather severe, cutting himself off from family and living as a hermit. Travis is moderate to severe. A nephew is rather high functioning. My grandsons whom I firmly believe both are on the spectrum....Alex is moderate to severe, and Evan is severely autistic. Even Darrin displays strong characteristics.
And gee as I type this I wonder why I often feel overwhelmed with gfgdom.
As to the eldest generation I'm personally aware of (husband's Dad and Uncle).....nurture had alot to do with their high functioning. They grew up and lived during a time where you either towed the line or else. And the "or else" wasn't at all pretty.
Same with husband's brother and cousins. (the cousins have the same parents) The one cousin did great, the other couldn't. Same parenting from good parents.
With husband and his older brother.......Older brother did better because more was expected from him. husband, not so good because mother in law, for whatever reason, tended to overcompensate for his short comings.
My nephew did well at least partially because he has an awesome Mom. My sister in law is a warrior Mom unto herself.
Travis, we been told, is likely much more severe due to the brain damage than he would've been otherwise. However, I look at the other males in the family and I find it hard to tell. Although Travis is far higher functioning than any of his neuro's and such would expect to see. Alex may be more moderate to severe for the same reason. Yet, I've not heard of anything that would lead me to suspect as the reason Little Evan is so much more severe, yet he is. And K has been using my parenting of Travis as a model to work from for her own boys. Then of course there is Darrin whom seems so far to be only displaying strong traits, but not the actual autism.
husband's family is actually quite interesting this way.
I think we, as parents, need to learn the limitations of the dxes of our kids and to accept them. (extremely hard) While also realizing that no one is going to follow a textbook example of any diagnosis, so they still have the potential of learning to make the right decisions ect. (where nurturing plays a huge part) But in the end, once they're adults and our job is done, it's up to them to take what they've learned and use it.