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<p>As one of the most important characteristics of human families, the number of coresident members (household size) varies widely in different historical times and cultures. However, whether an optimal household size and an evolutionarily stable household size exist remains to be theoretically and empirically tested. Here, we developed a theoretical model among relatives based on kin selection and game theory to investigate the interplay of household size and cooperation. We then use a decade of household dynamic data from a rural population in southwestern China (1,110 households) to test the associations between household size, household dynamics and cooperation. The theoretical analysis suggests the existence of an optimal household size and an evolutionarily stable household size (which are not necessarily the same) by considering the average local relatedness and the changes of the average inclusive fitness payoff of the family. The data analysis shows that average relatedness (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\:\stackrel{-}{r}\)</EquationSource> </InlineEquation>) decreases with increasing household size, and <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\:\stackrel{-}{r}\)</EquationSource> </InlineEquation>&#xa0;is associated with helping behaviour (especially for the elders) in the household; thus, the household tends to maintain a relatively high level of average relatedness by adjusting the number of coresident members through demographic events such as birth and moving out. These findings help explain the variation in household size in different cultures and demonstrate the importance of kin selection as an intrinsic factor in regulating family dynamics.</p>

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Kin Selection Underpins Family Dynamics in Rural China

  • 期刊论文

  • 发表日期：

    2026年3月9日

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  • Qiao-Qiao He,
  • Ming-Yang Wang,
  • Jia-Jia Wu,
  • Yi Tao,
  • Jie-Ru Yu,
  • Tian-Jiao Feng,
  • Song-Hua Tang,
  • Ling-Ling Deng,
  • Ruth Mace,
  • Xiu-Deng Zheng,
  • Ting Ji

摘要

As one of the most important characteristics of human families, the number of coresident members (household size) varies widely in different historical times and cultures. However, whether an optimal household size and an evolutionarily stable household size exist remains to be theoretically and empirically tested. Here, we developed a theoretical model among relatives based on kin selection and game theory to investigate the interplay of household size and cooperation. We then use a decade of household dynamic data from a rural population in southwestern China (1,110 households) to test the associations between household size, household dynamics and cooperation. The theoretical analysis suggests the existence of an optimal household size and an evolutionarily stable household size (which are not necessarily the same) by considering the average local relatedness and the changes of the average inclusive fitness payoff of the family. The data analysis shows that average relatedness ( \(\:\stackrel{-}{r}\) ) decreases with increasing household size, and \(\:\stackrel{-}{r}\)  is associated with helping behaviour (especially for the elders) in the household; thus, the household tends to maintain a relatively high level of average relatedness by adjusting the number of coresident members through demographic events such as birth and moving out. These findings help explain the variation in household size in different cultures and demonstrate the importance of kin selection as an intrinsic factor in regulating family dynamics.