Turns out there's a field of economics asking the same question. I read an interesting article in Ode Magazine, called "The altruism in economics". The article covers the blood donation scenario, along with something called the Ultimatum Game. In the game, two players are given a sum of money to share. Player A is to offer a portion of the sum to Player B. If B accepts, they share according to A's offer. If B rejects, both players get nothing. Traditional economics would say that A should offer B the minimum amount possible, since B should logically accept any offer. Interestingly though, the typical offer approaches a 50/50 split. It may be that humans have an inherent sense of fairness, or that Player A looks at it from B's perspective and (irrationally) decides that he should offer close to half in order to get B to accept.
In either case, it gives me hope for the future. When we finally realize that there is just one Earth, maybe we will learn to share.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiFaH-8E-FXw0E4bQnG2qZ00Cnv8WgH8R7J72X3-WfdFetpPLQLGw7P0ShMKa_nPguM7vAX13jLa0mufGBFQXQTg11Nrcm7d9WpJxJ-Ayl6tgcdup-5snCYcmJM-bzYTGnNrIH0og/s400/Sharing.jpg)