Baby Behaviour, ASD and Outcomes? Where's The Studies?

For all the studies undertaken in trying to understand Autism, I sometimes find myself frustrated by the aspects that are not explored. Or at least that I have not yet managed to uncover. The two big ones for me are the differences between children and adults on the spectrum who were clearly born ASD compared to those who were seen to develop normally then regress. And the differences between those as babies who were difficult versus those who were placid. And while I list these two as separate issues I expect there is some overlapping.

I raise this because these differences seem to point to more than temperament. The ASD babyhood story tends to go one of two ways. Either the one where parents rave about what an easy baby their child was. How he or she would happily lie in the bouncer for hours, never cried, and was generally low maintenance. Then there's the stories like mine with Harri. The baby that cries from the moment he is born and doesn't really stop from that point in. At least that's how it feels to the shell shocked parents. Initially colic is blamed, but then the three month mark comes and goes and you are still swinging, bouncing, carrying this crying baby, who rarely sleeps, needs constant motion, seems simultaneously over and under stimulated.

I felt quite alone in my parenting nightmare when Harri was a baby. No one could relate to what it was we were experiencing and advice about routines and sleep training were of no value, in fact they exacerbated my sense of bewilderment and frustration. This was my third child, so I knew something about caring for babies, but Harri had me baffled. Several trips to the doctors revealed no cause, and also no alarm bells. Just a fussy baby with a neurotic mother. A mother who got a label first, being Post Natal Anxiety.  And was packed off for counselling, which did not prove helpful because I still had this miserable child to return to each time. It took a label for Harri to put it all into perspective.

Imagine then my sense of validation when recently reading Born On A Blue Day by Daniel Tammet, an Autistic man with extraordinary savant talents. In it his parents recall Daniels early months,

"Yet my mothers first days with me at the hospital were not as she had imagined them. I cried constantly for hours at a time"
and
"Another means my parents found to relieve my crying was to give me the sensation of motion. My father regularly rocked me in his arms, sometimes for more than an hour at a time."
and
"In their despair they often put me in a blanket, my mother holding one end and my father the other and swung me from side to side. The repetition seemed to soothe me."

I could have written these very words for Harri. I am in no way suggesting he is a savant (or that I swung him around the room in a sheet as tempting as it was at times), in fact I am certain he is not, but the behaviours shared by Daniel and Harri as babies are remarkable in their similarity. I know there are many more stories out there like this and I cant help wonder how the children who started life this way tend to progress? Has there been any research that looks at the trajectory for kids who start out easy versus kids who like Harri and Daniel do not? What neurological differences account for some babies on the spectrum being able to happily absorb their surroundings and not feel the pressing need for motion, compared to those babies who appear overwhelmed while also physically under stimulated?

Harri is now a sensory seeker, he still loves the sense of motion, be it on a swing, or simply in the car. However he is not overwhelmed by sensory input the way many others on the spectrum are.  He can manage busy shopping centres with ease, in fact he enjoys them, he manages transition well for the most part, and can focus reasonably well for short periods of time considering he is not yet three. He is not the hyper sensitive, over stimulated, sensory overloaded kid he was. He has a bright trajectory.

There was no regression, he did hit his baby development milestones on time, albeit just, apart from speech. He was a kid clearly struggling from the get go. His story is so different from the ones where the child appears to be NT in every respect than backslides.  I know each person with ASD is different and that parent recall is notoriously inaccurate, but these startling differences are so distinct that surely they offer a key in understanding Autism better? Maybe I am being a non sciency simpleton here and missing something? But I am left mystified as to why the above issues do not generate more interest from researchers.

(I would be most thankful to anyone who could point me in the direction of any relevant research)