Thanks for the welcome. Okay, I will belatedly join the discussion, then.
I really enjoyed this book, although obviously enjoyed is not quite the right word, as it was also somewhat disturbing! I think part of the 'horror' element for me was in viewing a society which refused to accept there might be a problem with Ben. The (few) professionals consulted offered no understanding or even conception of Ben's condition, and therefore no potential insight or hope: a very bleak outlook.
Probably as a result of the parents' insistence that their child was some kind of throwback, or alien being, I was convinced that he was simply human, with extreme emotional, social & behavioural problems. For example, his father extends no attention to his son whatsoever (except for being the primary instrument in Ben's institutionalisation); he is kept locked away from his family overnight; he is kept shut away from the extended family whenever they visit. He is basically given very little opportunity to develop normal social & emotional skills, zero acceptance of self, and shown very little of what could be termed love, so of course all of these things combined would not lead to a 'normally' socialised little boy. By contrast, Amy (who is 'different' but in a different way) is initially treated by the extended family with wariness, but kept within the social unit, and ultimately flourishes.
Having read the spoiler somebody posted above from the interview with Doris Lessing I don't feel any less convinced of Ben's humanity. I don't believe the concept of the throwback versus the child with behavioural problems are necessarily mutually exclusive. And I think the fact that Lessing leaves Ben's condition undiagnosed and writes his story the way she does makes for a more ambiguous and successful story, overall.
Harriet and David built their idyllic life, with the assistance of and reliance upon their extended family. What I found interesting was the way this crumbled so readily in the face of a single instance of something slightly out of the ordinary: Ben. The extended family & support diminished to a most minimal level, and Harriet & David found themselves in direct opposition on how to balance the force of nature in their midst. Although they had craved parenthood, they did not really seem to comprehend the attendant responsibilities (like somebody above said, the decision to have a child is more usually a selfishly motivated act), and were not equipped to cope. They could not cope with their first four children without a lot of help. The manifestation of a child with even more demanding needs simply pushed them over the edge. I do believe that Harriet loved Ben and was motivated by more than guilt to care for him; but her care was not sufficient for his needs and came at the expense of her other childrens' needs.
Ben was the catalyst for the disintegration of David & Harriet's dream. This story was for me less about the fifth child himself, and more about his impact on the family unit around him, and the way this fell apart. I am of course fascinated by Ben, and am definitely going to be reading Ben in the World, as soon as my copy arrives. I will be intrigued to see life through his eyes rather than Harriet's.
I've heard of children before who simply observe what is going on around them for longer than is usual, but then begin speaking in full sentences. In Ben's instance, it just struck me as something which was typical of his 'difference' but not especially odd, in a more general sense.