Actually, my illustration (in my view, anyway) works perfectly. My point (such as it was) was about how people react defensively to psychological research, misinterpret it and then object to things that haven't been said.
The fact that grapefruit are on average bigger than oranges is so patently true that it's hardly worth stating. But what the statement does not deny, either, is that there may be some particularly large oranges and some particularly small grapefuit, such that despite the average there are some oranges which are bigger than some grapefruit. What that statement also doesn't deny in that in choosing what fruit to buy or eat, one might be thinking about other things besides size, which may not even come into consideration.
So, if that's what the research data indicate (and I haven't read the research itself) it may well be true that on average only children when they are children may be happier than children with siblings. But (despite some of the huffing and puffing on the other thread) that isn't anywhere close to saying that every only child is happier than every child with siblings, or even that every child with siblings is unhappy about it.
Equally, I am sure there will be some other measures on which children with siblings might well score higher than children without. To me, it seems self-evident that the slightly different life experiences which children with and without siblings have will mean that taken as a group they won't score identically on psychological tests and sometimes only children will be 'ahead' and sometimes those with siblings will be 'ahead'. But that says nothing about any individual child.
It would be sad if we had to abandon psychological research because the findings were uncomfortable. Research reports are like buses - there's another one coming along in a minute.