We have one young toddler daughter and are pondering when (and if) she should have a younger sibling -- and what age difference might work best for both of them.
For many (most) parents, what works for them is a major factor, but I have gone to a huge effort to prepare such that I'm ready to make whatever sacrifices I need to do what's best for my kids. I don't mind broadening the answers, but this question is specifically about the kid's angle.
There's a huge amount of soft "opiniony" discussion of this stuff in various media, but I'm more interested in whether people know of any really scientifically solid research that you think is useful to know (for example, data on stuff like 'success', not personality profiles); and what specific personal experiences people have that illustrate difficulties (the less obvious/cliché the better) of certain age differences.
(In our case, they will probably be primarily homeschooled... that probably changes some things, but the responses do not have to be limited to our situation.)
Do you have any siblings? Or children? What are the age differences? Can you reflect on any specific problems in your experience that an empathic parent might want to take into account?