Mace R
Department of Anthropology, University College London, UK.
Philos Trans R Soc Lond B Biol Sci. 1998 Mar 29;353(1367):389-97. doi: 10.1098/rstb.1998.0217.
Life history theory concerns the scheduling of births and the level of parental investment in each offspring. In most human societies the inheritance of wealth is an important part of parental investment. Patterns of wealth inheritance and other reproductive decisions, such as family size, would be expected to influence each other. Here I present an adaptive model of human reproductive decision-making, using a state-dependent dynamic model. Two decisions made by parents are considered: when to have another baby, and thus the pattern of reproduction through life; and how to allocate resources between children at the end of the parents' life. Optimal decision rules are those that maximize the number of grandchildren. Decisions are assumed to depend on the state of the parent, which is described at any time by two variables: number of living sons, and wealth. The dynamics of the model are based on a traditional African pastoralist system, but it is general enough to approximate to any means of subsistence where an increase in the amount of wealth owned increases the capacity for future production of resources. The model is used to show that, in the unpredictable environment of a traditional pastoralist society, high fertility and a biasing of wealth inheritance to a small number of children are frequently optimal. Most such societies are now undergoing a transition to lower fertility, known as the demographic transition. The effects on fertility and wealth inheritance strategies of reducing mortality risks, reducing the unpredictability of the environment and increasing the costs of raising children are explored. Reducing mortality has little effect on completed family sizes of living children or on the wealth they inherit. Increasing the costs of raising children decreases optimal fertility and increases the inheritance left to each child at each level of wealth, and has the potential to reduce fertility to very low levels. The results offer an explanation for why wealthy families are frequently also those with the smallest number of children in heterogeneous, post-transition societies.
生活史理论关注生育的时间安排以及父母对每个后代的投资水平。在大多数人类社会中，财富继承是父母投资的重要组成部分。财富继承模式和其他生育决策，如家庭规模，预计会相互影响。在此，我使用一个状态依赖动态模型提出了一个人类生育决策的适应性模型。考虑父母做出的两个决策：何时生育另一个孩子，从而确定一生的生育模式；以及在父母生命结束时如何在子女间分配资源。最优决策规则是那些能使孙辈数量最大化的规则。假设决策取决于父母的状态，该状态在任何时候都由两个变量描述：在世儿子的数量和财富。该模型的动态基于传统的非洲牧民系统，但它具有足够的通用性，可近似于任何一种生存方式，即拥有的财富量增加会提高未来资源生产的能力。该模型用于表明，在传统牧民社会不可预测的环境中，高生育率以及将财富继承偏向少数子女通常是最优的。现在大多数这样的社会正在经历向低生育率的转变，即人口转变。探讨了降低死亡风险、减少环境不可预测性以及增加抚养孩子成本对生育率和财富继承策略的影响。降低死亡率对在世子女的最终家庭规模或他们继承的财富影响不大。增加抚养孩子的成本会降低最优生育率，并增加在每个财富水平下留给每个孩子的遗产，并且有可能将生育率降低到非常低的水平。这些结果为为什么在转型后的异质社会中富裕家庭往往也是子女数量最少的家庭提供了解释。
