We in America are in one of the most individualistic societies, and capitalism being a system that allows you to go higher and higher up a potential pyramid, and the deal is that is comes with fewer and fewer safety nets. By definition the more stratified a society is, the fewer people you have as peers -- the fewer people with whom you have symmetrical reciprocal relationships; and instead all you have are differing spots in endless hierarchies, and a world in which you have few reciprocal partners is a world with a lot less altruism.
He makes some comments earlier that seem to idealize tribal society as peaceful and largely devoid of organized violence -- something I think Steven Pinker might take issue with given the data on the decline of violence over the course of human history (although "organized" may be a key distinction there). But I do think the quote above is interesting in light of the fact that countries with less economic stratification, such as the more socialist countries of Scandinavia, give significantly more to charity as a percentage of GDP than Americans do. But on the other hand, here in America where we have fewer social nets, people tend to give more voluntary. Granted, charitable giving is just one form of altruism, and it's interesting how, per the article in Forbes, altruism in countries with more egalitarian economic distribution may come in time, rather than money. How is it all connected? I'm not sure. But it's food for thought.