Goq,
I happen to know a number of families blessed with many children. North of a dozen. Anecdotally, your comment about a lack of individuality is way, way off base.
In these families that I personally know, the children all have distinct personalities and are accomplished in a number of ways. you could not in any way refer to them as group mentality, they each are so individual.
You are making an assumption, I am reporting my observations. And based on my observations, there is no basis at all to your suggestion.