My incredibly naive view on this is that with the first born the parents are full of energy and try their best to discipline (assuming the parents are not already dysfunctional human beings). When the next children come along they get progressively more tired and lax, or straight up angry and impatient. The younger kids then pick up these undesirable traits from their parents.

On the other end of the spectrum we have oldest siblings with some behavioral issues and the younger ones are like “nope, not gonna become like that myself”. I don’t feel like this group has the same controls as the other group from a study perspective.

Parents who maintain discipline with their kids through multiple kids are pretty rare and they are the unsung heroes of a civilization.

I have no sources on this except that it is this way in my family and the families of my parents friends.