My wife has the best answer to this. She always said we should have at least two children because otherwise the child would not have someone to conspire against his parents.

If that may be a truism, my data from my practice show me that an only child often has a harder time adapting to the great big world later in life than children who have siblings.

Humans, even small ones, tend to learn most from their peers, i.e. other children. So if there's another one around all the time, that's a learning sccelerator. If you are a close observer, you'll be amazed at the impact has. I know I am.

We now have three. Number three just turned three. He spoke fully grammatical sentences at two and potty-trained himself six months ago.