> If you keep this mentality going into adulthood you are most likely a psychopath.

You make it sound like this is selfish. The toddlers aren't expecting a favorable treatment to themselves, they expect others to be fair towards everyone else.

I'd argue the real damage happens once toddlers lose their altruistic mentality and get into the selfish competitive mentality we enforce today. Keep in mind, this only happens as a form of natural selection, as those with these traits have a better chance at survival/success in this environment.

It's fundamental to remember that better fit doesn't mean better overall. As I've read once here on HN (one of those life changing sentences that I happen to read here every once in a while), if you have a stupid environment, you get a stupid fit.