The Ultimatum Game is a pretty well-tested experiment. I put you and Joe in a room, and give Joe ten one-dollar bills. He is allowed to propose a split of the money with you (and that's all he's allowed to do). You are allowed to accept the split, or reject it. If you accept, that's how you split the money, and you both go home richer. If you reject it, neither of you gets anything. You only play once.
Now, if you're an entirely independent, totally rational economic being, you'll accept any offer that gives you money. It's free money! But that's not what happens in most cultures. Splitting at 50-50 is always accepted. Splits where the 'decider' gets less money are rejected more and more often as the split gets less favorable.
In places where money is a relative oddity - among tribespeoples - this has been tried a couple of times (not enough for any kind of certainty, but still). And there, uneven splits were offered and accepted almost universally.
If that's so, then this isn't an inborn feature of our thinking. It's something we've learned - an absorbed (or consistently individually created) principle that is deeply fundamental. I'm not going to try and codify this principle; plenty of attempts have been made to do so, and they almost all end up showing off the biases of the describer, rather than revealing absolutes. Instead, I'd rather look at what it might mean.
It seems to indicate that people who deal in money, an abstract commodity at the best of times, have at least one gut-level learned response that benefits the social body. It's not in your rational interests to reject money outside of a 'fair' split, but it's in the interests of society for this kind of thing to be going around.
This is in line with some other aspects of cognition - puzzles that ask you to "catch the cheater" are often easier to solve than abstract version of the same puzzle. It's possible that we come into the world wired or disposed for this kind of thinking, and thus pick it up very quickly. It's possible that it's built into social subtext so often that we can't help but catch it, even if it's invisible to us when we do. There are, again, plenty of theories.
What matters, to my eye, is that we can build on responses like this, or tear them down. There's a fairness principle down there, and it can be expanded on and spoken to.