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.
In my experience, usually one child of a set seems to always be a lot more demanding than others.
I'm now experiencing this with my own children to some extent. We have 2 kids so far, a 4 year old boy and a 1 year(and a couple months) old girl. They both wake up around 6:30 in the morning. And whereas before that meant that I'd never get to sleep in past that time, now our son just gathers a bunch of toys, climbs into his sister's crib, and they play until like 8am before they finally get bored and start calling for us. So at least as far as mornings go, 2 kids has proven easier than 1(after that initial 6 months from hell period where you don't get any sleep at all that is)
Can you go into this a bit more? I have a single child, and just thinking of trying to raise another one makes me feel like my lungs are deflating.
(If this isn't a good venue for this conversation, you can email me - my email is in my profile.)
By the time our third came, I shrugged it off as "well I've already got two... what's a third?"
If I found out we were having a forth, I'd probably feel the same.
The first is the worst, because now your life is restricted by a child. The second has a small impact, because now it takes two parents to chase the kids. The third has a small impact, because now they can run in 3 different directions.
After that though, it is no big deal. The only thing you notice is when you have to upgrade to a larger vehicle. For me this is now a van with 5 rows of seats.
Everything gets more efficient as the family gets larger. If your vehicle choice drops from 30 MPG to 15 MPG as you go from 4 to 10 people, the per-person numbers have gone from 120 to 150. It's like that for lots of things: cost of a house, time spent shopping, etc. You get a bulk discount. Things purchased for the first kid can be used for the next. The tax situation improves too. With enough kids, you probably won't have to pay any income tax.
With just one, you have already lost your ability to wander freely and be left in peace. You mostly can't lose that more. At this point, you might as well enjoy the benefits of having a huge family.
It's really weird.