Most human societies exhibit a distinct class structure, with an elite, middle classes, and a bottom class, whereas animals form simple dominance hierarchies in which individuals with higher fighting ability do not appear to form coalitions to “oppress” weaker individuals. Here, we extend our model of primate coalitions and find that a division into a bottom class and an upper class is inevitable whenever fitness-enhancing resources, such as food or real estate, are exploitable or tradable and the members of the bottom class cannot easily leave the group. The model predicts that the bottom class has a near flat, low payoff and always comprises at least half the society. The upper class may subdivide into one or more middle class(es), resulting in improved payoff for the topmost members (elite). The model predicts that the bottom class on its own is incapable of mounting effective counter-coalitions against the upper class, except when receiving support from dissatisfied members of the middle class(es). Such counter-coalitions can be prevented by keeping the payoff to the lowest-ranked members of the middle classes (through concessions) well above that of the bottom class. This simple model explains why classes are also absent in nomadic hunter-gatherers and predominate in (though are not limited to) societies that produce and store food. Its results also agree well with various other known features of societies with classes.

中文翻译：

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为什么在人类中会发生阶级形成，但在其他灵长类中却不会发生：灵长类联盟模型。

大多数人类社会表现出独特的阶级结构，包括精英阶层，中产阶级和底层阶级，而动物则形成简单的统治等级制度，在这些等级制度中，具有较高战斗力的人似乎没有形成联盟来“压迫”较弱的人。在这里，我们扩展了灵长类动物联盟的模型，发现每当有适应性提高的资源（例如食物或房地产）可开发或可交易且底层成员无法轻易地将其分为底层和上层阶级时，不可避免的离开小组。该模型预测，底层阶级的收入几乎是平坦的，回报率很低，并且至少占社会的一半。上层阶级可以细分为一个或多个中层阶级，从而为最高层成员（精英）带来更高的回报。该模型预测，除非得到中产阶级不满成员的支持，下层阶级本身就无法与上层阶级建立有效的反联盟。通过使中产阶级中排名最低的成员（通过让步）的报酬远高于底层阶级的报酬，可以防止这种反联盟。这个简单的模型解释了为什么在游牧的狩猎者和采集者中也没有阶级，而在（尽管不限于）生产和储存食物的社会中占主导地位的原因。其结果也与阶级社会的其他各种特征相吻合。通过使中产阶级中排名最低的成员（通过让步）的报酬远高于底层阶级的报酬，可以防止这种反联盟。这个简单的模型解释了为什么在游牧的狩猎者和采集者中也没有阶级，而在（尽管不限于）生产和储存食物的社会中占主导地位的原因。其结果也与阶级社会的其他各种特征相吻合。通过使中产阶级中排名最低的成员（通过让步）的报酬远高于底层阶级的报酬，可以防止这种反联盟。这个简单的模型解释了为什么在游牧的狩猎者和采集者中也没有阶级，而在（尽管不限于）生产和储存食物的社会中占主导地位的原因。其结果也与阶级社会的其他各种特征相吻合。