Autism (Asperger Syndrome at that time) was suggested by a colleague when I described my oldest daughter when she was 8, and she was diagnosed at 9. Precocious speech and reading, development otherwise normal, very sociable, absolutely fine at playgroup and nursery and socially with other children as a preschooler. Nothing to distinguish her from any other intelligent little girl. Went to school, became That Child, tremendous behavioural problems, aggressive towards other children and teachers, screaming meltdowns, destructive. Her behaviour at home also deteriorated. "It's YOUR fault, you should do this, you should do that," her father and I were told again and again, and of course we believed this because we WANTED to feel that we could make a difference. It was such a relief when the speech and language therapist said, "No, you're NOT bad parents," and in my own professional role I make a point of saying to parents, "I believe you."
At the time there was "none of that in OUR family" on both sides. Within months her younger brother had also been diagnosed as having autism. Several cousins on my side have also subsequently been diagnosed as having autism.
The diagnosis didn't give us access to much support back in the late 1990s, but it gave us some understanding of why, for example, another child's repetitive sniffing might result in DD1 trashing the classroom and why she couldn't eat in the dining room with the other children. She spent a couple of years out of mainstream education and had a rough time until her mid teens, but came out the other side and has done well in adulthood.