What this analysis of self-interest completely misses from real life is that someone's self-interest doesn't just apply to them, but to everyone around them. People have to automatically consider their family, friends, work, and community in every decision, and in all of their behavior. They can't buy a Ninja sportsbike if their kids are hungry, unless they are an asshole. If they don't consider others when they make economic decisions they could suffer some serious consequences, particularly others breaking off contact with them. The only adults that this doesn't apply to are the homeless