Sharing can arise without generosity, because defending a surplus costs more than it is worth to the holder and less than it is worth to the taker.
Blurton Jones's model requires no altruism and no reciprocity: a hunter with more meat than he can eat faces diminishing value per portion, while a hungry neighbour faces high value, so the neighbour will fight harder for it than the holder will fight to keep it. Sharing is the equilibrium. The account explains why sharing norms are strongest for large, perishable, unpredictable resources.
In practice
- A team lets another team take spare capacity because reserving it would cost more argument than it is worth.
- A household that would defend its car does not defend the leftovers, and calls the second hospitality.