I was very nearly a one-child family from choice... and now have two, with a big age gap! So I have sort of seen both sides of this and have no interest in either criticizing or defending one-child families.
At the individual level, growing up as an only child doesn't seem to have any particular impact on a child's personality. Only children do tend to have slightly different relationships with their parents. But this does not seem to impact the relationships they build with others. Kids behave differently at home compared to with their peers.
At the societal level, a move towards smaller families will have some impacts on societysome positive, some negative. For environmental reasons, it's actually imperative that people have smaller familieshowever, we should be mindful of the potential pitfalls and downsides that the "one-child" shift can have at the macro level. I don't want to live in a society like South Korea, let me put it that way.
I once saw an interesting chart which looked at "family size" (0 kids, 1 kid, 2 kids, 3 kids, 4 kids etc.--I'll see if I can find it) and correlated this with people's views on how much heredity influenced people's personality traits. The fewer kids people have, the more emphasis people put on upbringing; the more kids they had, the more people de-emphasized upbringing and stressed the importance of heredity. Probably mainly because people have more than one kid, attempt to do the same things yet see their several kids turning out surprisingly different.
Although I have only 2 kids and almost stopped at one, I have to say that I have really enjoyed the relatively chilled perspectives which you tend to hear from parents of multiple children. We need to be careful that as families get smaller, we don't end up with ever more intense and competitive parenting, with everyone obsessed with the idea that every emotional or psychological issue a child has is caused by their parents (or, conversely, congratulating themselves on their child's success rather than understanding that a lot of this is about luck).
There's more. Smaller families allows parents to put more resources into each child--expensive extra curriculars, tutoring, consumer items, holiday camps, birthday parties and so on. I see no evidence that this "spoils" only children at all, but at the macro level it does subtly up the game for everyone else. It's hard to be chilled about your children's frugal upbringing of free activities and "playing out" if you are surrounded by families who seem to be putting their children in more and more expensive, paid activities and enrichment stuff every year. We are all influenced by our peers.
Tutoring and educational expenses, in particular, are a concern because there is an "arms-race" factor here, and because overdoing that kind of stuff can start to have a really negative effect on children and on family life. There are only a limited number of top university places and top jobs available, so as more families start tutoring and Kumoning and paying for private educational enrichment in all sorts of ways, it is difficult for other families to say no to this as wellwe all get caught up in the arms-race, sooner or later. Small families are probably connected with this in a cyclical kind of wayhaving one child increases the "eggs in one basket" feeling and pushes parents to focus more on their child's education, and of course parents have more resources to spend per child. It is no accident that the world's top performing education countries mostly have very low birthrates.
South Korea is the perfect example of a society which has engineered itself into a seemingly permanent one-child trap. People started having smaller families to focus on kids' education, which then became a sort of circle. Smaller families then had more resources to spend on tutoring. Tutoring centers (hagwons) have upped their prices, knowing families have more to spend. This means it's now impossible to succeed in Korean education without tutoring because of the competition/arms-race factor. So everyone has to tutor like mad, meaning you now do not have the choice of having a big family even if you wanted one. Now that South Korea is into the second generation of small families, it's continuing to get more intense, because you now have one kid to four grandparents in a lot of families, increasingly the amount that can be used to bankroll the education process per child. South Koreans know that their education system is horrible for families and is making kids miserable, but nobody knows how to get off the hamster wheel now.
This is an extreme example, but it's something we should think about. And it is nothing to do with one-child families doing something "bad"but rather, something which we should be cautious about at the societal level. If we can be aware of the potential pitfalls of a society-wide move towards more one-child families, we can start thinking about things we could do to prevent this from happeninglike changing the way our educational system functions, or publicizing more widely the actual scientific studies that have been on parenting and personality (I am talking about stuff like twin studies and adoptee studies, which suggest that while parenting styles can teach their children skills such as educational stuff, they do not have much influence on actual personality development).