In many indigenous societies, people are categorized into several cultural groups, or clans. People in the same clans believe to share common ancestors. Marriage possibility is determined by clan attributions, e.g. a man in clan A should find his wife from clan B but not C or D.  Such rules between clans constitute kinship structures. Although anthropologists have revealed their structural patterns, it remains unsolved how they are organized and what explains their diversity. This study investigates the origins of kinship structures by building a simple model of society. In the model, people have cultural traits and mating preferences. Their values determine the marriage possibilities and regulate cooperation with cultural siblings and in-laws, and competition with mating rivals. Here, I followed the ethnographic observations. Through an evolutionary simulation with the pressures to increase cooperators and reduce competitors, societies are spontaneously segmented into clans and kinship structures are organized. Diversity of evolved kinship structures, including clan endogamy, dual organization, and generalized and restricted exchange, are explained by the adaptation to different environmental conditions of the necessity of cooperation and avoiding competition. A statistical analysis of a global ethnographic database supports the results on the environmental dependence of kinship structures. It is now possible to explain macroscopic structures of kinship based on microscopic interactions of people, their environmental dependence, and the historical causality of their evolution.

References:

Claude Lévi-Strauss. "The Elementary Structures of Kinship", 1949.

Kenji Itao and Kunihiko Kaneko. “Evolution of kinship structures driven by marriage tie and competition.” Proceedings of the National Academy of Sciences 117.5 (2020): 2378-2384.

Kenji Itao and Kunihiko Kaneko. “Emergence of kinship structures and descent systems: multi-level evolutionary simulation and empirical data analysis.” Proceedings of the Royal Society B 289.1969 (2022):1-10 .

Kenji Itao and Kunihiko Kaneko. “Formation of human kinship structures depending on population size and cultural mutation rate.” Proceedings of the National Academy of Sciences 121.33 (2024): 1-9. 

Gift relationships establish social relations as well as the transference of goods in many human societies. Recipients of gift should reciprocate to the donor. If reciprocation is completed, they become friends. Unless, recipients become subordinate to the donor. The totality of such social relations constitutes the network. Social networks characterizes different types of social organizations, including bands of small kin groups, tribes with moderate inequalities, and hierarchically organized chiefdoms. However, factors and mechanisms that cause the transition between these types have hardly been explained. Here, we focus on the gift as the driving force for such changes. In this study, I build the model by idealizing gift interactions and simulating the consequent social change due to long-term massive interactions. In the model, people give their assets to each other, produce them, and reciprocate for the gift. Gift and reciprocation strengthen their relationships. Through simulation, we demonstrate that as the frequency of gift increases, economic and social disparities arise successively. Simultaneously, network structures shift from bands to tribes and to chiefdoms. Statistical analysis using the Standard Cross-Cultural Sample, a global ethnographic database, empirically verifies theoretical results.

References:

Marcel Mauss. “The Gift: Forms and Functions of Exchange in Archaic Societies”, 1925.

Elman Service. “Primitive social organization: an evolutionary perspective”, 1962.

Kenji Itao and Kunihiko Kaneko. “Transition of social organizations driven by gift relationship.” Humanities and Social Sciences Communications 10.1 (2023):1-10.

Kenji Itao and Kunihiko Kaneko. “Emergence of economic and social disparities through competitive gift-giving.” PLOS Complex Systems 1.1 (2024) 1-16.

Families form the basis of society, and anthropologists have observed and characterized a wide range of family systems. Demographers revealed correlation of characteristics in family systems and those in social structures or ideologies. This study developed a multi-level evolutionary model of pre-industrial agricultural societies to simulate the evolution of family systems and determine how each of them adapts to environmental conditions and forms a characteristic socio-economic structure. In the model, competing societies evolve, which themselves comprise multiple evolving families that grow through family labour. Each family has two strategy parameters: the time children leave the parental home and the distribution of inheritance among siblings. The evolution of these parameters demonstrates that four basic family systems emerge; families can become either nuclear or extended, and have either an equal or strongly biased inheritance distribution. Nuclear families in which children leave the parental home upon marriage emerge where land resources are sufficient, whereas extended families in which children staying at the parental home emerge where land resources are limited. Equal inheritance emerges where the amount of wealth required for a family to survive is large, whereas strongly biased inheritance emerges where the required wealth is small. Analyses on the wealth distribution of families demonstrate a higher level of poverty among people in extended families, and that the accumulation of wealth is accelerated in families with strongly biased inheritance. By comparing wealth distributions in the model with historical data, family systems are associated with characteristic economic structures and then, modern social ideologies. Empirical data analyses using the cross-cultural ethnographic database verify the theoretical relationship between the environmental conditions, family systems, and socio-economic structures discussed in the model.

References:

Emmanuel Todd. “The Explanation of Ideology: Family Structure & Social Systems”, 1985

Kenji Itao and Kunihiko Kaneko. “Evolution of family systems and resultant socio-economic structures.” Humanities and Social Sciences Communications 8.1 (2021):1-11