This is really interesting. I will have to get the book (father of a daughter, here). But I have to wonder how much of this correlation is not about the presence or absence of a father, but the presence or absence of a second well-invested parent regardless of gender. Or even the presence or absence of other close family members. For instance, what's the average age of menarche in traditional hunter-gatherer societies where none of the kids are raised soley by one or two parents but by the whole village/group/clan/whatever? Single moms simply don't, by and large, have the time to provide as much care as two parents. I'm not accusing the authors of being on a traditional marriage platform, but I am dubious of how important it is that the second strong parent be male.