I think you're probably right about this (not based on first-hand experience of having a child, mind - I haven't), but I can't quite see what it's doing here. Is this meant to be some sort of objection to the comment you're replying to? It isn't obviously in tension with it.
I can believe that that's true for a significant portion of humanity
Just explaining why it is certainly true for the vast majority of people who actually have had children.
There is a lot of research on this topic. I know later studies contradict earlier studies, but I'm not sure who to believe here. Wondering if anyone can help.
In case it isn't clear, the basic question is the effect of having children on happiness.