Your third sentence seems to agree with my suspicion - subsequent children get less attention. (Though I reiterate my belief that some of the apparent developm

Your third sentence seems to agree with my suspicion - subsequent children get less attention. (Though I reiterate my belief that some of the apparent developmental difference is because second children don't need to be as communicative as the first child.) The sibling comments here primarily echoed a similar sentiment, while agreeing that there's variation and assuming that's just random, while also tacitly confirming that 2nd-child tended to perform less well.