This entry was written by Rowena McPhee as part of a project done in BIAN 2133 ‘Human Reproductive
Strategies’ at The Australian National University in 2019 Semester 2.
Introduction
Fertility rates are determined by an array of ecological, evolutionary, economic and cultural factors, and vary markedly between populations. In general, post/demographic transition societies tend to have lower fertility rates, while pre-demographic transition societies tend to have higher fertility rates; this trend may seem counterintuitive when considering resource allocation trade-offs. I will begin this essay by providing a brief explanation of demographic transition theory, followed by some background information pertaining to the evolution of reproductive strategies. I will then evaluate the quality-quantity trade-off argument as an explanation for fertility rate variation.
Main Text
The Demographic Transition
The demographic transition is a social scientific process that models a society’s transition from experiencing high mortality and fertility rates to low mortality and fertility rates (Bongaarts, 2009). The model involves four phases: initially, societies experience low economic development, a high mortality rate, and a high fertility rate. In the second phase, an increase in technology and development causes the society’s mortality rate to rapidly decline, while the fertility rate remains high. The third phase then sees the fertility rate decline due to factors such as increased economic opportunities, health care and education. In the fourth stage, post-demographic transition societies experience low fertility and mortality rates, and high levels of development. The changes in fertility rate over the course of the demographic transition result in changes in family size; pre-demographic transition societies tend to have larger families than post-demographic transition societies.
Life History & Optimality
Fertility rates are dependent on many ecological and evolutionary factors, such reproductive strategy, resource abundance, and trade-off between quantity and quality of offspring. Evolutionary theory suggests that optimal fertility rates will be determined by these factors in such a way that maximises the biological fitness of the parents.
Humans are iteroparous, in that we experience multiple reproductive events over the course of our lives. This reproductive strategy leads humans to face a trade-off dilemma – how should we allocate our reproductive resources in order to maximise fitness? One possibility is to produce a large number of offspring, each of whom would receive a smaller proportion of the available resources, and may therefore be of lesser quality. Alternatively, prospective parents may decide to limit the number of offspring they have, enabling them to provide each child with more resources, therefore increasing the quality of the offspring. This dilemma is known as the quality-quantity trade-off; individuals must decide how to allocate their resources in order to achieve the fitness-maximising optimal family size.
A surface-level analysis may lead to the assumption that more abundant resources would result in higher fertility. That is, parents would be able to have a great number of children without sacrificing the quality of their offspring, as they have enough resources to provide adequately for many children. However, evidence suggests that humans do not follow this pattern (Hill and Reeve, 2004). As the demographic transition suggests, the trend of human fertility follows the opposite pattern; fertility is much higher in societies that have fewer resources, and much lower in societies that have more resources (Nargund, 2009).
Evolutionary Explanation of Low Fertility
Human fertility does not fit these seemingly intuitive predictions of resource allocation and optimal family size. However, this does not mean that there is not an evolutionary explanation for human fertility. Human fertility is likely a conditional strategy that has evolved to maximise fitness in a range of different circumstances (Sanderson & Debrow, 2000).
In a situation in which there are fewer resources – such as a pre-demographic transition agriculturalist society with low economic development – there is lower certainty of survival to reproductive age due to high infant and child mortality rates. Fitness will only be maximised if offspring are able to reproduce, so the fitness maximising reproductive strategy would be to have a large number of children, in the hopes that some of them would survive to reproductive age. The large number of offspring, combined with the relative scarcity of resources, means that each child would be of ‘low quality’; however, this is unimportant if they are still ultimately able to reproduce.
Alternatively, in a situation in which there are more resources – such as a post-demographic transition industrialist society with high economic development – there is a higher certainty of survival to reproductive age due to low infant and child mortality rates. In this scenario, it is highly likely that all children will survive to reproductive age, so the fitness maximising reproductive strategy would be to have a small number of children, and to invest the available resources heavily in those few offspring. In having a lower quantity of offspring, the parents are able to increase the quality of their children via intense resource investment. Furthermore, strong economic development opens up opportunities beyond reproductive activities, such that parents may choose to pursue education, employment or other activities instead of producing more offspring; there is an altered trade-off between reproductive activities and non-reproductive activities.
This model of quality-quantity trade-offs offers some explanation for the low fertility rates observed in post-demographic transition societies; however, it is not adequate on its own. Lawson et al (2012) show that the fertility rates observed in developed societies fall far short of the evolutionary optimum, even when accounting for the quality-quantity trade-off model discussed previously. As such, there must be some other factors influencing fertility rates. For example, Goodman et al (2012) suggest that low fertility rates could be biologically maladaptive, but socially adaptive, as a result of cultural evolution. Low fertility rates in post-demographic transition societies can be jointly explained by the quantity-quality trade-off of family size, and a cultural evolutionary preference for small families.
Conclusion
Fertility rates vary significantly between pre- and post-demographic transition societies, with more developed societies exhibiting lower fertility rates than less developed societies. This variation can be attributed to the changes in quality-quantity trade-offs in different circumstances, in addition to some contribution from cultural evolution.
References
Bongaarts, J. (2009). Human population growth and the demographic transition. Philosophical Transactions of the Royal Society B, 364, 2985.
Goodman, A., Koupil, I., & Lawson, D. (2012). Low fertility increases descendant socioeconomic position but reduces long-term fitness in a modern post-industrial society. Proceedings of the Royal Society B, 279, 4342-4351.
Hill, S., & Reeve, H. (2004). Low fertility in humans as the evolutionary outcome of snowballing resources games. Behavioural Ecology, 16(2), 398-402.
Lawson, D., Alvergne, A., & Gibson, M. (2012). The life-history trade-off between fertility and child survival. Proceedings of the Royal Society B, 279, 4755-4764.
Nargund, G. (2009). Declining birth rate in Developed Countries: A radical policy re-think is required. Facts, Views and Vision in Obstetrics and Gynaecology, 1(3), 191-193.
Sanderson, S., & Dubrow, J. (2000). Fertility Decline in the Modern World and in the Original Demographic Transition: Testing Three Theories with Cross-National Data. Population and Environment: A Journal of Interdisciplinary Studies, 21(6), 511-516.
