I am a single child. I think my parents put all of their energy into me. My father worked long hours and my mother had her own business and always worked. They would not have had any extra energy for another child. They were also poor, and if there were more children they would have struggled more. I also think there was always a combination of relief and guilt about having one child. Neither had good relationships with their parents or siblings, so they were relieved that I did not grow up with siblings. They felt guilty that I had no brothers or sisters at the same time.
Megan-bright makes a good point. Parenting is not something that occurs completely naturally. With subsequent children, the learning curve becomes smaller. This being said, lots of positive things have been said for single child parents, but if these good things occur with these parents, these same parents would provide the same good things for multiple children. It is not the number of children that enables parents to be good, quality parents.
I believe that when the child is an infant or toddler the parents, assuming that they are both first time parents, may have more anxiety, doubts, lots of questions, and more fear in regards to common ailments the child may be experiencing. The parents may in turn be more overprotective and respond to the child's every issue without giving him/her much time to make mistakes and fall down at times.
This could in turn, cause the child to expect a lot of immediate and full attention, as they are the only child with the parents' full attention.
I do believe that there are no cut and dry answers, as the parents' working schedules, personalities, and beliefs must also be taken into consideration.
I also believe that as time goes on, the parents relax more and adjust to parenthood, and this would leave no or minimum significant differences between their child and a child with multiple siblings.
I agree with post #4 in that when you have your second child you are a lot more relaxed in your parenting style. I know of couples who have chosen to have only one child for whatever reason, my observation of their parenting behaviors compared to parents with multiple children is that they sometimes seem more consumed with their child's life. They tend to try and give the child everything they want.
I can speak to how MY behavior changed when I went from one child to two. The first child is like the tester child. We were not only quite protective of her when she was young, but we worried about all sorts of little things. We were constantly comparing eating/sleeping/behavioral habits with others, trying to figure out what was "normal." I rarely let her out of my sight as well, constantly fearing that if I left her alone something bad would happen.
Then I had my second.
This one has been to the doctor a total of one time in almost 2 years (outside of regular checkups). I do not question unusual eating, sleeping, behavior anymore, because, either I've seen it before, or I just know that it will change in a couple weeks.
I think parents who only have one child act differently because they have no points of comparison except other people's children, and what they read or hear. They also have more time and energy (like PPs say) to devote to just one.
We have become far more laid back with our second child (and I imagine this will grow even more if we have a 3rd or 4th). My own parents were most strict/rigid with my older brother and me (the 1st and 2nd kids) and it seemed like my 2 younger sisters were spoiled and got away with everything.
Single child parents have more time and energy to devote to the life of only one child. That doesn't mean they actually DO spend more time or energy, it just means it is more possible for them. In an ideal situation, one child would have all the attention her or she needs. A single child never gets dragged along to a sibling's activities; never has to share homework time with a sibling who also needs help; never has to share the parent's attention at play time. Single child parents never have to find a way to balance the needs of more than one child.
This is a stereotyped answer, because I am sure that not all parents of a single child act like this. However, we tend to say that the parents of a single child tend to be more protective of the child. They have only one child and so they put all of their emotional and financial resources into that child. They want to make everything perfect for the child.
Again, this is surely not true of all parents of one child.
It is generally believed that all single children are pampered, lonely, or selfish. These are just prejudices. Children's personality develops according to the education they receive.
Children who have siblings are getting along better with siblings than with parents, whereas a single child spends more time in the company of adults. The result is that the single child will copy the behavior and the speech of adults, becoming more rational and more prepared to face the life. This, surely, is a good thing.