I’ve tended to notice that Harvard classes are dominated by people who are the eldest born. (You can do a survey and at any point in any upper-echelon school system, you’ll get two-thirds be eldest children)
Does that mean that the eldest siblings are the smarter ones? As much as I wish that were true (I am an eldest son), this post from EconLog points out why that is faulty logic (and another why correlation does not equate to causation):
If you regress real income on birth order, you get the same pattern as my wife’s law school class. The first-born averages $1900 more than the second-born, who averages $1900 more than the third-born, and so on.However, if you regress real income on birth order AND family size, you get a totally different picture. Birth order makes essentially no difference (in fact, the sign reverses), but average income falls by about $2400/child in your family. First-born only child? You’ll make more than average. First-child child in a big family? You’ll do no better than the fifth-born child – maybe a little worse!
Does this show that big families hurt incomes? Possibly, but the simpler story is more plausible: Poor people have more kids, and kids of poor people tend to be poor themselves.