Substantial theoretical and empirical evidence demonstrates that fertility entails economic, physiological, and demographic trade-offs. The existence of trade-offs suggests that fitness should be maximized by an intermediate level of fertility, but this hypothesis has not had much support in the human life-history literature. We suggest that the difficulty of finding intermediate optima may be a function of the way fitness is calculated. Evolutionary analyses of human behavior typically use lifetime reproductive success as their fitness criterion. This fitness measure implicitly assumes that women are indifferent to the timing of reproduction and that they are risk-neutral in their reproductive decision-making. In this paper, we offer an alternative, easily-calculated fitness measure that accounts for differences in reproductive timing and yields clear preferences in the face of risky reproductive decision-making. Using historical demographic data from a genealogically-detailed dataset from 19th century Utah, we show that this measure is highly concave with respect to reproductive effort. This result has three major implications: (1) if births are properly timed, a lower-fertility reproductive strategy can have the same fitness as a high-fertility strategy, (2) intermediate optima are far more likely using fitness measures that are strongly concave with respect to effort, (3) we expect mothers to have strong investment preferences with respect to the risk inherent in reproduction.

中文翻译：

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生育率的边际估值。

大量理论和经验证据表明，生育需要经济、生理和人口权衡。权衡的存在表明，应该通过中等生育水平来最大化适应度，但这一假设在人类生活史文献中并没有得到太多支持。我们建议找到中间最优值的难度可能与计算适应度的方式有关。人类行为的进化分析通常使用终生繁殖成功作为其适应度标准。这种适合度测量暗含地假设女性对生育时间漠不关心，并且她们在生育决策中是风险中性的。在本文中，我们提供了一个替代方案，易于计算的适应度测量，可解释生殖时间的差异，并在面临风险生殖决策时产生明确的偏好。使用来自 19 世纪犹他州的详细谱系数据集的历史人口统计数据，我们表明该度量在生殖努力方面是高度凹的。这一结果具有三个主要含义：(1) 如果出生时间适当，低生育率的生殖策略可以与高生育率策略具有相同的适合度，(2) 中间最优更有可能使用强凹的适合度测量关于努力，（3）我们预计母亲对生育固有的风险有强烈的投资偏好。使用来自 19 世纪犹他州的详细谱系数据集的历史人口统计数据，我们表明该度量在生殖努力方面是高度凹的。这一结果具有三个主要含义：(1) 如果出生时间适当，低生育率的生殖策略可以与高生育率策略具有相同的适合度，(2) 中间最优更有可能使用强凹的适合度测量关于努力，（3）我们预计母亲对生育固有的风险有强烈的投资偏好。使用来自 19 世纪犹他州的详细谱系数据集的历史人口统计数据，我们表明该度量在生殖努力方面是高度凹的。这一结果具有三个主要含义：(1) 如果出生时间适当，低生育率的生殖策略可以与高生育率策略具有相同的适合度，(2) 中间最优更有可能使用强凹的适合度测量关于努力，（3）我们预计母亲对生育固有的风险有强烈的投资偏好。