Formation of human kinship structures depending on population size and cultural mutation rate

What is it about?

This study theoretically uncovers the mathematical mechanisms behind the formation of human kinship structures. In traditional societies, people are often divided into clans with specific marriage rules, such as men in Clan A marrying women in Clan B. The research shows that these structures can spontaneously form through a combination of kin and in-law cooperation, as well as sexual rivalry. The effects of population size (N) and cultural mutation rate (μ) on the formation of these structures are detailed. In the model, families' growth rates increase when they have more kin, in-laws, and mate candidates, and fewer rivals. To enhance cooperation, families align to become kin and marry non-kin. To reduce rivals, they diversify their marital preferences, leading to an increase in the number of clans. These dynamics result in the formation of distinct clans united by marriage ties, creating kinship structures. Analytical calculations reveal that the explorability of complex structures depends on Nμ², while their stability depends on μ²/N. This suggests that larger populations are more likely to foster complex structures, though they are also influenced by the mode of cultural transmission.

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Photo by Jayesh Jalodara on Unsplash

Photo by Jayesh Jalodara on Unsplash

Why is it important?

The study of kinship structures is fundamental in cultural anthropology. Anthropologists have observed a wide variety of human kinship structures. While this diversity has been classified into a few classes, the processes driving their evolution and the conditions determining their classes were previously unknown. This study provides a mechanism for the evolution of these structures and highlights how environmental conditions influence their classes. Furthermore, the study offers a generalizable mechanism for the emergence of social complexity. It is often assumed that human social structures become more complex as population size increases. However, a detailed mechanistic explanation for the relationship between population size and social complexity has been lacking. Human kinship structures serve as a fundamental example where people are divided into interdependent groups necessary for reproduction. These structures are widely observed in traditional societies and minimally exemplify the interdependency between differentiated groups. Studying the evolution of kinship structures allows for the exploration of a general mechanism for the formation of interdependent groups, along with their dependence on N and μ. The study reveals that kinship structures depend on the values of Nμ² and μ²/N, which determine the explorability and maintainability of complex structures, respectively. Such proportional relationships between parameters are known as "scaling laws" in statistical physics. Scaling laws serve as indicators of universality; when distinct phenomena share common scaling behaviors, they may be governed by similar mechanisms. In this sense, the findings are considered generalizable and fundamental to the study of social evolution.

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