Only child syndrome:
It's a strange quality that is by no means exclusive to only children. However, in my personal experience I've seen it more in those without siblings.
We've all seen it: those people who think they're the center of the universe. Ever since their birth, they have been the sole receiver of their parents' attention. They've had gifts and praise and love showered upon them freely. They get what they want, and when they don't get, they are completely shocked, or throw a fit (or both). They are very unique individuals, and feel a deep need to be the center of everyone else's attention.
Children with siblings, on the other hand, often observe their siblingless peers with great envy. With siblings around, children have to compete for their parents' attention. Material goods have to be shared. Praise is only sprinkled, and likewise must be shared with brothers and sisters. Rivalry runs rampant. These children have to learn how to interact with their peers early on: they play games together, share toys, or at least just peacefully ignore the others' existence.
The grass is always greener on the other side, they say. I have siblings, and many's the time I've wished to have been an only child. One of my closest friends is an only child, and has always wanted a sibling.
I am not advocating that either situation is better than the other, or that one produces a better individual than the other. However, it appears that people with siblings are better equipped to deal with society "in the real world" beyond their own family, while only children may have a difficult time particularly in school, which may lead to other issues in socializing further down the road.