Being an only child seems to be a bit psychologically damaging. I could conjecture for pages here, but suffice it to say I believe one is better off having 2-3 children or none at all. Sure there can be situations like being raised closely with cousins or friends that are similarly beneficial, but kids absolutely must have close companions near their own age -- they don't seem to fully learn to be empathetic otherwise.
This is only the opinion of one person, take it however you wish.