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. 


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.)