I well remember a young child with AS coming to stay for a week. Right little sh*t at home but he was not allowed chips, only once (as his stay extended to 10 days), with meals. Neither did he get any pudding unless he ate up his main course - food not eaten, therefore not hungry - and by the end of the first week (initially he was made clearly aware that he could/would go home if he misbehaved, but that clear implication, of bad behaviour, was not repeated, even once during his stay) he was eating runner beans, boiled tats and almost anything else put on his plate.
He did not always want ice cream for pudding either. Junk food such as potato crisps and similar were off the menu, too, as were fizzy drinks (which were his usual liquid intake).
A remarkable change in his mood over the period he was with us. He behaved well with no tantrums and carried on sensible conversation on a variety of subjects. He regressed very quickly when he went home, I believe, but meals of crisps and other junk food while sitting watching the telly was not particularly helpful.
It has been noted that a 'mad' hound (collie cross) that used to run around the tops of the furniture, bark, yap and bite was also transformed, when exported to another home and fed, and treated, properly. It is stiil going along quietly after several more years (was about to be put down if the move had not taken place). This was from the same household.
This leads me to believe it is the domestic situation that is the cause in many instances - I would not say all, but many - of these so called 'syndromes'. Probably the reason why most 'grow out of it' when they eventually grow up and realise, and learn, the proper values and correct way they are expcted to behave in a wider society.
RAB