A truly discouraging thing to me is how easily humans see the world as dichotomized between Us and Them. This comes through in all sorts of ways —social anthropology, lord of the flies, prison experiments, linguistics (all those cultures where the word for the members of that culture translates into "People," thus making a contrast with the non-people living in the next valley).
As a neurobiologist, I'm particularly impressed with and discouraged by one finding relevant to this. There's a part of the brain called the amygdala that has lots to do with fear and anxiety and aggression. Functional brain imaging studies of humans show that the amygdala becomes metabolically active when we look at a scary face (even when the face is flashed up so quickly that we aren't consciously aware of seeing it). And some recent work—solid, done by top people, independently replicated — suggests that the amygdala can become activated when we view the face of someone from another race. The Them as scary, and the Them being someone whose skin color is real different from our own.
Damn, that's an upsetting finding.
But right on the heels of those studies are follow-ups showing that the picture is more complicated. The "Other skin color = scared activated amygdala = the Other" can be modified by experience. "Experience," can be how diverse of a world you grew up in. More diversity, and the amygdala is likely to become activated in that circumstance. And also, "experience," can be whether, shortly before your amygdala is put through the brain imaging paces, you are subtly biased to think about people categorically or as individuals. If you're cued towards individuating, your amygdala doesn't light up.
Thus, it seems quite plausible to me that we are hard-wired towards making Us/Them distinctions and not being all that nice to the Them. But what is anything but hard-wired is who counts as an Us and as a Them —we are so easily manipulated into changing those categories.
So, I'm optimistic that with the right sort of priorities and human engineering (whatever that phrase means), we can be biased towards making Us/Them dichotomies far more benign than they tend to be now. Say, by making all of us collectively feel like an Us with Them being the space aliens that may attack us some day. Or making the Them to be mean, shitty, intolerant people without compassion.
But, I'm sure not optimistic that we'll soon be having political, religious or cultural leaders likely to move us effectively in that direction. Just to deflate that optimism.