I think your selection bias theory makes sense.

Humans had to evolve to be both nurturing and caring, but also cold and ruthless. We needed to be nurturing and caring of our helpless young, but cold and ruthless towards animals that we needed to kill for food. Where other people lie on that spectrum can be all over the map. On one side you have what right-wingers call "bleeding-heart liberals" who love every human and often love animals too. I admit I'm pretty close to that pole, but I kill animals for meat and with a lot of effort and reason I can get myself to see a group of people as an enemy (but with people, it is hard to not care about them). Then you get nationalist Republicans who only care about their nation. At the extreme you have religious-pseudoracial supremicists with zero concern for anyone outside of their tribe, and who even have a penchant for screwing each other over and being proud of it.

I think a good balance is being kind to anyone willing to be kind back, treat every stranger like family and they will treat you the same most of the time. If you discover that was a mistake, recategorize them into the untrustworthy camp and avoid them (without trying to make them an enemy). Play tit for tat but with random forgiveness... the game theory shows that without random forgiveness you end up in a never-ending tit-for-tat defect battle like what Israel and Palestine are doing. If either side doesnt practice forgiveness from time to time it will never work.