Not my own theory, can't remember whose, but I go with the theory that in every society people are arranged along a bell shaped curve (with, as the name implies, most people somewhere towards the middle, some kinder or more self-centered than others). To the left are the hawks, self centered, willing to take advantage of others. To the far right, the Mother Teresa's. If everyone were well behaved and trusting, the hawks would have an advantage and their genes would propagate. Too many hawks, the society becomes dysfunctional except for the minority doves who begin to strengthen in number. Over time, the center ebbs and flows to the right and left, but never too far.
It is possible that the Polynesian societies, having to live on small islands, were centered more towards the right (the doves) than Western societies, but they would have been easy pickings for a hawk among the group. Thus, there still would have been a span of behavior. As far as the boys marooned on the island, given that there were only six of them and they were friends, they could all have been more dovish in nature. Thus their success.