This entry was written by Gabrielle Eyer as part of a project done in BIAN 2133 ‘Human Reproductive
Strategies’ at The Australian National University in 2019 Semester 2.
Introduction
Life history theory uses optimality models to study the optimal solution to trade-offs between various components of life history strategy. This theory explains that trade-offs between reproductive strategies maximise the fitness, or reproductive success, of an individual (Lawson and Borgerhoff Mulder, 2016). In mating and parenting strategies, these trade-offs include the effort, resources, time, or any energetic allocation that an individual invests in present and future offspring (Walker et al., 2007). One principle, proposed by David Lack, attempts to explain mating and parenting strategies as a trade-off between offspring quantity and quality (Lawson and Borgerhoff Mulder, 2016). While the optimisation of an individual’s fitness remains at the forefront of human reproductive strategies, the energy ratio allocated to offspring quality over quantity has shifted. This change in the offspring quantity-quality trade-off in WEIRD societies can be explained by the decrease of offspring quantity, due to the increased offspring survival and the delay in reproduction, and the increase in offspring quality via increased financial parental investment.
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Decreased Offspring Quantity
The offspring quality-quantity trade-off life history theory can be used to explain the upper limits of female fertility, suggesting that offspring quality and survival into reproductive age is an important influence of human reproductive behaviours (Zietsch et al., 2014). This theory can be observed in humans via evolved physiological constraints related to female fertility such as singleton births, long gestation periods and lactation amenorrhea. Each of these constraints restrict the maximum number of offspring a female can birth without compromising the current offspring’s growth and development (Lawson and Borgerhoff Mulder, 2016). Despite this trade-off explaining the upper limits of human reproductive patterns, recent studies have shown that post-industrial, WEIRD populations have an average family size below the predicted optimality (Lawson and Borgerhoff Mulder, 2016). This deviation from optimality can be explained by a shift of energy allocated towards offspring quality over quantity.
Greater Offspring Survival to Reproductive Age
From an evolutionary perspective, the decrease in offspring quantity in WEIRD populations is suggested to be related to the increased survival of offspring to reproductive age. A child’s survival to reproductive age is an important factor in human reproductive strategies (Zietsch et al., 2014). In pre-industrial societies, high child mortality rates resulted in a limited number of offspring attaining reproductive age, and thus increasing offspring quantity was a means to maximise an individual’s reproductive success. In modern WEIRD societies, the low rate of childhood mortality means that more children reach reproductive age and thus an individual’s reproductive success can be attained at a lower cost (Lawson and Borgerhoff Mulder, 2016). In addition, as a result of industrialisation, external environmental risks, such as the prevalence of diseases, have declined leading to a greater certainty of survival of offspring (Lawson and Mace, 2010), and thus there is no need for upbringing a greater number of offspring to ensure future reproduction. Based on the offspring quantity-quality trade-off, the reduction of cost in offspring quantity provides an opportunity for greater offspring quality, as a means to maximise an individual’s reproductive success.
Later Reproduction
Many social factors have influenced the shift in the offspring quantity-quality life history trade-off. Mating and parenting, particularly for females is an immense allocation of energy, time and resources. In post-industrial, WEIRD societies, the decision to have children imposes a trade-off of a woman’s quality of life, whether by impeding on a female’s education, career or health (Lawson and Mace, 2010). Along with decreasing overall offspring quantity, the timing of fertility in females has changed. Females are having children at a later time in their reproductive years, therefore reducing their maximal potential number of offspring (Lawson and Borgerhoff Mulder, 2016). Additionally, prioritisation of higher education and advancement in one’s career over rearing children has become a social standard in WEIRD societies. While education may come at a fertility cost, thus contributing the reduction in offspring number, it is believed that childbearing impedes on an individual’s education more so than the reverse (Lawson and Borgerhoff Mulder, 2016). Other factors that contribute to the delay of female fertility may involve strategies used in securing a high-quality mate, as well as the evolution of marriage norms that have led to a shift in the offspring quantity-quality trade-off. Because biparental care and marriage are common in WEIRD societies, offspring quantity is dependent on the reproductive strategies of two individuals rather than one, which could result in conflict and delay in reproduction (Lawson and Borgerhoff Mulder, 2016). Overall, social norms can highly influence human reproductive patterns. In WEIRD populations, these norms have led to delaying the timing of reproduction, which has contributed to the overall decrease in offspring quantity.
Increased Offspring Quality
As well as reducing offspring quantity, WEIRD populations are investing more into the quality of their offspring. In WEIRD societies, the perceived costs of child-rearing have increased significantly, leading to reduced fertility (Lawson and Borgerhoff Mulder, 2016). One of the ways financial parental investment has shifted is in the increase in contribution to the offspring’s education (Lawson and Borgerhoff Mulder, 2016). While it would be expected that wealthier individuals would be able to financially invest more towards a greater number of offspring, the opposite is observed, whereby increased economic success in a family results in fewer offspring, but greater investment in each individual. It is proposed that as income increases, the quality of offspring is maximised rather than the quantity (Doepke, 2015). Instead of gaining reproductive success by the increased number of offspring, in a post-industrialised WEIRD society, the fitness of a parent is more heavily influenced by the quality of the offspring.
Conclusion
While principles of maximising individual fitness still apply to the evolution of reproductive patterns, the adaptive nature of the quantity-quality trade-off is a result of the optimal regulation of this trade-off, such that in modern WEIRD populations, offspring quantity is reduced and offspring quality is increased, rather than the overall maximisation of offspring number. It is not yet known if low fertility in modern WEIRD societies is adaptive in the long-term, but with the completion of multigenerational studies, the adaptiveness of low fertility could be better understood (Lawson and Mace, 2010). While the evolutionary trade-off between offspring quality and quantity continues to be relevant in modern WEIRD societies, there are many other trade-offs that could also explain the societal shift in the optimisation of offspring quality and quantity.
Literature Cited
Doepke, M. (2015). Gary Becker on the quantity and quality of children. Journal Of Demographic Economics, 81(1), 59-66.
Lawson, D., & Borgerhoff Mulder, M. (2016). The offspring quantity–quality trade-off and human fertility variation. Philosophical Transactions Of The Royal Society B: Biological Sciences, 371(1692), 20150145.
Lawson, D., & Mace, R. (2010). Optimizing Modern Family Size. Human Nature, 21(1), 39-61.
Walker, R., Gurven, M., Burger, O., & Hamilton, M. (2007). The trade-off between number and size of offspring in humans and other primates. Proceedings Of The Royal Society B: Biological Sciences, 275(1636), 827-834.
Zietsch, B., Kuja-Halkola, R., Walum, H., & Verweij, K. (2014). Perfect genetic correlation between number of offspring and grandoffspring in an industrialized human population. Proceedings Of The National Academy Of Sciences, 111(3), 1032-1036.
