If you do a survey of students at an elite college, you’ll likely see a strong overrepresentation by those who are the eldest sibling in their family.
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 this is faulty logic:
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 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.