Selfishness
This is a myth — that the only child in the family «whether we like it or not» grows up as an egoist. It all depends on the upbringing and attitude of the parents to his «uniqueness». I am an only child, and such collectivists are still to be found. I have one child who took care of all the kids in the yard and perfectly found a common language with the elders. And the families I know, where there are several children, are closed to the family. They have enough mental and material strength only for their relatives.
The family is not the only place of education. At school and in the yard among many children and further in society among people, the values of mutual assistance and team spirit are easily perceived, if you are lucky and there is someone to teach this.
Families with many children are very different. Children are often given overwhelming responsibilities early on and growing up, they do not know how to exist in normal relationships, because they did not live through certain situations in childhood for which there was no time.
By the way, most families with any number of children are not friendly at all, children do not care about each other sincerely, because they do not see this spiritual relationship from their parents.
And the younger ones throw those same tantrums to the older ones.