Because while I don't have maternal instincts of any kind (and anyway, that would be a creepy way to relate to a partner, right?) I do have an insatiable desire every now and again to tickle someone until they cry or to hold someone down and force-feed them leaves. And it's funny, I can see the same instincts in Frank, who is otherwise a completely non-violent human being. It takes another eldest to know how to defend against the cruel tendencies of eldest children.
Anyway, I'm sure this is a thing that psychologists have spent lots of time thinking about, and I fully realize that my tiny pool of anecdata is not scientifically meaningful. Still, it's an odd thing to see in action. What do you guys think? Same birth order or mixed birth order? Or does it matter?