Different family systems around the world developed due to a predictable pattern of environmental and social pressures, according to a newly designed mathematical model incorporating decades of results from anthropologists, demographers and historians. In addition to providing a theoretical framework to understand social evolution, the researchers hope their technique will be the foundation of a "universal anthropology" field of research.
The study, published in Humanities and Social Sciences Communications, was completed by first-year doctoral student Kenji Itao and Professor Kunihiko Kaneko, an expert in a theoretical biology who explores universal aspects of complex living systems, from the University of Tokyo Graduate School of Arts and Sciences.
Traditionally, anthropologists and demographers have focused on the relationships between spouses, siblings, and parents and children to characterize family systems. However, how these personal relationships are shaped by larger social and environmental factors is not fully understood in traditional anthropological approaches.
"We can observe four types of family systems all over the world. Why is this so?" said Kaneko.
Those four systems are pairs of extended or nuclear families with wealth inherited either equally among all siblings or biased towards only one sibling.
"For example, in countries like China or Russia, anthropologists observe big, extended families where kids continue living with their parents as adults and wealth is equally inherited among siblings. Germany and Japan have extended families with biased wealth inheritance. France traditionally has nuclear families with equal wealth inheritance. England has nuclear families with biased wealth inheritance," Kaneko explained.
"These different family systems evolved while adapting to environmental conditions, specifically the availability of land and the necessity of wealth for survival," said Itao.
Itao constructed an abstract model of hypothetical agricultural societies and ran simulations with different amounts of wealth and land resources needed and available for survival. The model then calculated the different family systems that evolve in response to those different conditions.
Itao's mathematical model demonstrated the same four family systems observed by real-world anthropologists, revealing the environmental pressures that create these family systems.
The nuclear family system arises when land is plentiful, while the extended family arises when land resources are scarce. When the wealth required for a family's survival is small, exclusive inheritance only by one child evolved. When families need large amounts of wealth to survive, inheritance is equal among all siblings.
The researchers used an ethnographic database of 186 societies around the world to statistically compare their model's results to documented patterns of real family systems.
"This allows us to empirically justify our theoretical results," said Itao.
"We expect these results can provide a framework to explain the relationship between family system and social ideology through economic structure, but of course real-world socio-economic policies are never automatically determined according to family system," said Kaneko.
"The model was built using traditional or historical family systems. We think some new points should be discussed about new family systems developing now and in the future," said Itao.
For now, the results serve as a guideline for understanding the universal physical and financial pressures that shape social and economic realities for families all over the world. Through the analysis of general abstract models, the researchers aim to reveal universal features in human societies, intending to build the new field of "universal anthropology" research.
