Family Time and birth order

The Post ran a really interesting story this past weekend about how firstborns get much more quality time with their parents than later-born siblings.  The article discusses the possibility that this quality-time differential may help explain studies that show that firstborns are more likely to be successful adults than later-born siblings (as for me, since my sister is 8-years older, that makes me pretty much like a firstborn).  Anyway, what I found most interesting about the story was this accompanying graph:

What really stuck out to me, was that you are going to get a lot more parental attention as the first of several children than if you are an only child.  Why?  The article does not really address this issue, so I thought I would use this forum to go with my theory.  Basically, I think you are seeing a selection bias.  On average, persons who choose to have multiple children are simply more into being parents than those who stop at one.  Obviously, this does not apply to everyone, but enough to make some big statistical differences.  One interesting way to test my theory would be to look at the numbers for those persons who had only one child, but had wanted more (fertility problems, etc.).  While most persons who are parents really love and enjoy being parents and wanted to be parents, that is surely not the case for all.  Some people aren't going to figure out that parenting really is not for them until after they've had that first child.  Others, perhaps were somewhat ambivalent but finally give in to the extensive social pressure of “so, when are you going to have a baby.”  Once that barrier is crossed, there's a lot less pressure along the lines of “so, when are you going to have another baby.”  These persons, naturally, would spend less time with that firstborn that those multiple-child parents who really love parenting. 

Anyway, so, that's my naive theory on the matter.  I'd love some feedback on this one.  And, if you should happen to be one of my friends who have only one child (and you know who you are), I realize of course that you are among the most dedicated parents I know and that surely none of my comments apply to you.