Evolution of family systems and resultant socio-economic structures

Families are fundamental societal units, and their structures have been extensively studied by anthropologists. Family traits, including residence and inheritance patterns, are relatively stable, influenced by cultural transmission and adaptation to environmental conditions. While existing research demonstrates correlations between family traits and socio-ecological factors, the causal relationship remains unclear. This paper investigates the interplay between ecological conditions, family systems (specifically residence and inheritance patterns), and societal economic structures using a novel multi-level evolutionary model. The model focuses on pre-industrial agricultural societies, where family systems significantly impact land use and inheritance. The model incorporates elements like diminishing returns to labor, family labor contributing to wealth production, and cultural inheritance of family traits with slight mutations. The multi-level evolutionary framework simulates competition between families and societies, allowing for the emergence of distinct family systems based on adaptive strategies. This approach aims to bridge evolutionary anthropology, demography, and socioeconomic history to understand the interplay between family systems and social structures.

Existing research in anthropology and history has examined the diversity of family systems and their relationship with socio-ecological conditions. Studies have focused on cultural transmission, adaptation to social and ecological environments, and the influence of factors such as the Western Church on the emergence of nuclear families. Evolutionary anthropology has explored family relationships through parental investment theory and intra-family competition for reproductive resources. Quantitative studies have linked family traits to subsistence patterns and socio-ecological conditions, and phylogenetic comparative analyses have examined the origin and historical change of family traits. Despite these advancements, the causal relationships between family traits and social factors remain debated. This study tackles this issue by exploring a multi-level model to study the evolution of family systems in pre-industrial agricultural societies, thereby integrating understanding from evolutionary anthropology, demography, and socioeconomic histories.

The researchers employed an agent-based multi-level evolutionary model simulating pre-industrial agricultural societies. Societies comprised families, each characterized by population, wealth, and two strategy parameters: λ (inheritance inequality among siblings) and s (probability of children remaining in the parental home). The model incorporates factors like wealth accumulation through family labor, diminishing returns to labor, and environmental perturbations affecting societal survival. Societies split when family numbers double, and another society is randomly removed to maintain a constant number of societies (N). Families reproduce based on wealth (following a Poisson distribution), and strategy parameters are inherited with minor mutations. The model simulates two levels of competition: between families within a society and between societies. Environmental parameters, land capacity (c), and wealth required for survival (e), influence the evolution of family strategies. Simulations were performed for various environmental conditions and parameters, assessing the long-term evolution of family systems. The model was extended to include marriage, allowing the examination of son-biased investment and polygyny. Wealth distributions were analyzed to characterize social structures, fitting to power-law and exponential distributions. Finally, empirical analysis using the Standard Cross-Cultural Sample (SCCS) database was conducted to verify the model's predictions.

Simulations revealed four distinct family systems emerging based on land capacity (c) and wealth required for survival (e): 1. **Stem families:** (extended families, unequal inheritance) evolve under conditions of low land capacity and low wealth requirements. 2. **Community families:** (extended families, equal inheritance) emerge with low land capacity and high wealth requirements. 3. **Absolute nuclear families:** (nuclear families, unequal inheritance) are favored with high land capacity and low wealth requirements. 4. **Egalitarian nuclear families:** (nuclear families, equal inheritance) evolve under high land capacity and high wealth requirements. The model shows that extended families exhibit higher poverty rates, while unequal inheritance leads to faster wealth accumulation by heirs. Wealth distributions across societies generally followed a power law for the poor and an exponential distribution for the rich, with tail heaviness influenced by environmental parameters. The analysis of the SCCS database corroborated the model's predictions: Land scarcity correlated with extended families, while frequent internal warfare correlated with equal inheritance. Furthermore, the number of poor people correlated positively with extended families, and the number of rich people negatively correlated with equal inheritance, demonstrating a clear link between family system and wealth distribution.

The model successfully integrates anthropological and historical perspectives, showing how family-level strategies drive societal-level economic structures. The emergent family systems align with historical observations in different regions of Eurasia, where land availability and societal pressures shaped the prevalent family structures. The model's predictions concerning wealth distributions also align with empirical data, supporting the link between family systems and economic inequality. The impact of family structure on societal development is evident from the connection between family systems and socio-economic structures, potentially providing a framework for understanding the evolution of various political ideologies. The model helps explain why different family systems and wealth distribution patterns have historically emerged under different environmental conditions.

This study presents a multi-level evolutionary model demonstrating the emergence of four distinct family systems and their associated socio-economic structures based on environmental conditions. The findings integrate anthropological insights on family-level behavior with macro-level historical observations, highlighting the significant impact of family structures on societal development and wealth distribution. Future research could enhance the model by incorporating factors like social stratification, explicit intra-family competition, and broader subsistence strategies beyond agriculture to broaden understanding of the complex interactions of family systems, socio-economic factors, and historical development.

The model's limitations include its focus on pre-industrial agricultural societies, excluding the complexities of social stratification and intra-family competition. The empirical analysis relies on correlations from the SCCS database, without providing a definitive causal relationship between family systems and environmental factors or historical dynamics. Further research is needed to expand the model's scope, incorporating these factors to fully grasp the complex interplay shaping societal evolution.