I'm a psychology professor, so I have a tendency to look at things from a different perspective than most. :) The first thing that went through my mind when I first read your question is that the farther apart children are born, the higher each child's intelligence is. This has been shown over and over in numerous studies. It has to do with the amount of attention (intellectual stimulation) each child gets. When children are born close together, you're dealing with two children who are very young, and therefore, very demanding. Each child isn't going to get the same amount of attention as they would if they were either only children or 3 or 4 years apart.
Obviously there are always exceptions and people have children close together all of the time. Just thought I'd give you a different perspective to think about.
at 7:31 PM on Aug. 21, 2009