Coming into this a bit late, but just wanted to comment on the belief that only children can be spoilt & selfish, etc.
My 5 yr old son is an only child and is definitely not spoilt or selfish. Well, ALL kids can be ego-centric to a certain extent (as my ds can), but he is not 'selfish' - he shares his possessions brilliantly and he knows very well that he doesn't get a new toy or a treat every time we go shopping. He is very good when I say 'NO' to something he wants, because he doesn't expect to get it. We only buy him things as treats or for special occasions, like birthdays etc. He is compassionate and caring and loves other kids to bits.
Yes I am a bit protective of him but it's simply because he's my first AND only child, so I suppose that comes with the territory. He is definitely not coddled though, and I encourage him to do things on his own and try new things.
I think the only thing he doesn't/won't understand are the sibling dynamics other children experience, as other people have also mentioned in this thread. He tends to relate to adults in quite a natural & confident way because he is around them a lot, and perhaps this may go against him at some stage? But at the moment, he has a lot of little friends & playmates who he relates to just as well as any other children. He will obviously miss out on having siblings of a similar age which I am sad about, but I just wanted to make the point that 'only' children are not necessarily spoilt, selfish or mal-adjusted. All depends on the family environment, I'd say.