To continue this line of reasoning, it seems to me that in a healthy child, his sense of fairness expands with his sense of personhood. As a toddler, children are typically extremely self-centered, and part of what they learn in their third year of life is that they are not after all the center of the universe. As children come to internalize that other people are indeed people to the same degree that the child himself is a person, the concept of 'fair' also applies to those people, and so increases in the complexity of its application.
In general, it seems to be the process of differentiation.
When the child is born, it is "one" with the mother. The mother is merely an extension of itself. In such a mindset, "fair" does not exist because everything "is" the child.
As the child ages, it becomes aware that the mother is a separate entity (hopefully) and begins the detachment process (resulting in the "roving" behavior where the child will wander from the mother figure if he feels secure, but needs to come back constantly for reassurance).
The more differentiation occurs, the more that "fairness" can come into play. You cannot be fair if you are essentially solipsist in nature.
What's sort of funny is the cycle that people go through. At first, we think everything is "us" and so take care of "us." Then we differentiate and exclude those who are "not us." Then we come back around (or perhaps we never really left?) and broaden our relational web so that others are incorporated into "us" as independent individuals. (Isn't that what love is? Treating other people as we'd treat ourselves, rather than excluding them as Other? There is "us" -- not "me." And the broader the web, the more philanthropic or loving the person?)
The check on this would be to ascertain whether development of the concept of fairness is arrested in people who never make that leap of understanding that other people are in fact people. Does the autistic person or the psychopath (and I DO NOT mean to equate the two) have a modified or stunted sense of what is 'fair' due to his or her condition?
I do not know. Maybe Cafe or someone else can answer that more adequately?
I think sensitivity to others -- the ability to see them as part of "us" while still remaining unique and independent -- results in fairness and even actions of love.