Yeah, this rings true from experiencing two kids at my home and visiting single children and multi-children families as a kid. The single children where very d

Yeah, this rings true from experiencing two kids at my home and visiting single children and multi-children families as a kid. The single children where very demanding on their parents. The only source of attention and entertainment is the parents, so this makes sense. Two children entertain themselves a bit more, but like in duopolic situations you have the most extreme competition. This leads to a lot of fights over shared resources. With 3+ children fights over resources between 2 children don't lead to zero sum gains, so that the competition actually becomes less. Cooperation becomes more important. And the kids almost always entertain themselves. I'd definitely say the 3+ children parents were usually the least stressed, but maybe that was just my impression. The older siblings at some point functioned as quasi parents to the younger siblings.