I would much rather help a friend or group of friends do well as opposed to randoms or people I don't like. First, I like my friends and want them to be happy (and am going to be the one who hears about it when they aren't), so I have a vested interest in helping them achieve happiness and success whereas I have a weak interest in the happiness and success of randoms as we have no relationship. Second, if I can help my friends become happy and successful, they become better able to help me become happy and successful (by pooling resources, providing advice, providing introductions, creating mutually beneficial opportunities, creating happiness by spending time with me, providing trust and facilitating vulnerability, providing capital or expertise etc). This sets up a positive feedback loop and creates strong ROI for my efforts. I get net negative ROI from helping a random (assuming no relationship will develop and I expend time or funds) so it's far less likely that I will use my scarce time and resources to do it.
The entire world runs on relationships, favors, and reputation.
We do a lot more for people we know and like and trust than for randoms; I don’t believe this is restricted to just a few cultures. It shows in nature as well, unless there’s a massive amount of anthropomorphism going on in those studies. Rather more likely, it seems there would be selection pressure in favor of helping your offspring and those close to you.
There's a line between "is it OK to start a start-up with your dorm-mate?" and "is it OK to spend 5% of the nation's budget buying submarines through your brother-in-law's banana company?"
You have information about your dorm-mate, and you aren't acting on behalf of others. But if the same norms applied to CEOs and judges, we could not have our civilisation.
This is too broad. Is it really so? I believe that we don't want to disadvantage ourselves relative to other groups. But that we want to gain advantage at somebody's else expense - not sure.