Treating people as they deserve.  



Fairness comes in different varieties, which may work together:  



Fairness and cooperation reinforce each other.  Cooperation may be described as what you put in to a situation; fairness refers to what you get out of it.  If people are treated fairly (whatever that means in the context) then they are more motivated to cooperate.  

In modern “primitive” societies of hunter-gatherers, there is egalitarianism (a very flat power structure with all members of the group contributing to decisions) and distribution according to need, so that all members of the group are cared for.  

Our nearest relatives, the comparatively unevolved chimpanzees and bonobos, do not share like humans do; typically, high-ranking individuals dominate the proceedings and eat first.  It is thought that egalitarian sharing in the human way evolved because we started scavenging and capturing large prey animals where there was a lot of meat to go around and cooperation was necessary to secure a carcass or kill the prey.  



Natural selection is relative.