Very vaguely related, I had a somewhat-friend in college who told me about her, her twin, and her year younger sister would fight constantly all the time. Imagine 3 close aged kindergardeners just constantly at each other's throats when you were really not ready. They were so fed up that they went into the kitchen placed the three of them in equal distances away from the center, and then gave them each a knife and said "Go! If you hate each other so much, kill each other!"

The all started sobbing and hugged each other, and got along a lot better after that.

That's...def trauma territory, but, it's an example that human instinct to divided resources (emotional attention from a parent is REQUIRED for children's psyche) isn't darwinian. America specifically touts Survival of The Fittest as THE default human psyche and I find that it's just not true.