Abstract
The idea that trade-offs between offspring quantity and quality shape reproductive behaviour has long been central to economic perspectives on fertility. It also has a parallel and richer theoretical foundation in evolutionary ecology. We review the application of the quantity–quality trade-off concept to human reproduction, emphasizing distinctions between clutch size and lifetime fertility, and the wider set of forces contributing to fertility variation in iteroparous and sexually reproducing species like our own. We then argue that in settings approximating human evolutionary history, several factors limit costly sibling competition. Consequently, while the optimization of quantity–quality trade-offs undoubtedly shaped the evolution of human physiology setting the upper limits of reproduction, we argue it plays a modest role in accounting for socio-ecological and individual variation in fertility. Only upon entering the demographic transition can fertility limitation be clearly interpreted as strategically orientated to advancing offspring quality via increased parental investment per child, with low fertility increasing descendant socio-economic success, although not reproductive success. We conclude that existing economic and evolutionary literature has often overemphasized the centrality of quantity–quality trade-offs to human fertility variation and advocate for the development of more holistic frameworks encompassing alternative life-history trade-offs and the evolved mechanisms guiding their resolution.
Highlights
The intuitive concept of a trade-off between offspring quantity and quality is central to theories of human fertility with an independent origin in both economics and evolutionary ecology
Goodman et al [5] demonstrate that low fertility is associated with superior descendant school performance, educational attainment, and adult income across four generations of a large Swedish cohort born during the demographic transition
We argue that Lack’s original focus on clutch size rather than lifetime fertility, in addition to economic demography’s focus on the demographic transition rather than variation in pre-demographic transition fertility, has led much of the existing literature to overemphasize the pivotal role of the offspring quantity–quality trade-off in accounting for human fertility variation
Summary
The intuitive concept of a trade-off between offspring quantity and quality is central to theories of human fertility (i.e. number of births) with an independent origin in both economics and evolutionary ecology. Adaptive lag is certainly not unique to humans (see Schlaepfer et al on ‘evolutionary traps’ [38]) it is anticipated to be severe in human populations that have undergone the rapid and dramatic social, economic, and demographic changes accompanying the dawn of agriculture and the industrial revolution [39], and it opens up new possibilities of using culturally transmitted information to afford adaptation [40] Both life-history theory and economic accounts of fertility, along with related sociological models of family size and achievement [41,42], are united in their shared emphasis on the dilution of parental investment as the dominant factor dictating relationships between offspring quantity and quality. Selection acts on the net product of all of the pathways through which offspring number and well-being are related, thereby shaping reproductive strategies
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