The Quantum Effect of Fertility
Apologies for the downtime. I have been working hard to finish the fourth chapter in my long-running project on demographics. I am finally ready to present this work. I have titled the chapter “Into the Wilderness” because that is, in a theoretical sense, where we must go to properly understand what drives fertility in a modern context, and in the transition from a traditional society to a society characterised by rapid economic and technological growth and development. I have also updated the list of references, which can be found on the landing page for the project, with links to PDFs where available. If you are the kind of person that prefers to digest information in bite-sizes, I have copied by Twitter thread summary below the introduction and link to the paper.
Whether you’re an evolutionary biologist, cultural sociologist or a neoclassical economist, the study of human fertility behaviour can be boiled down to an interplay between two immovable forces: the quantum and tempo effect. The first treats the fundamental question of reproduction; how many children to have, and how much resources to invest in each of them. In its simplest form, the quantum effect is the study of how much, if at all, women exert control over the quantity of offspring they produce. The extent to which they do—and almost all disciplines agree that they do in most social contexts—the analysis focuses on the conditions that determine the number of children, and how much resources that are devoted to each of them. It is an analysis of trade-offs, concentrated on the trade-off between the quantity and quality of offspring. How this balance is achieved represents one of the most crucial processes in the study of reproduction, aggregate fertility, and the demographic transition.
Do you want to read the rest? Click here for the PDF.
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Twitter-thread:
My latest article in my demographics project explores the drivers of falling fertility during the DT, and why below-replacement fertility emerge with such regularity. The quantum effect attempts to answer these questions. 1/19
This is a multidisciplinary enquiry. Understanding the quantum effect requires familiarity with sources from evolutionary anthropology/biology, economics, and as we move on to tempo effects, the softer social sciences. 2/19
Vining (1986) asks whether modern fertility trends are maladaptive, for two reasons: first, people tend to voluntarily reduce birth rates below rates consistent with fitness maximisation and secondly, the positive link between wealth/income and fertility reverse in modernity. 3/19
Mulder (1998) recaps the flurry identifying three potential evolutionary explanations for what appears to be non-fitness maximising low fertility 1) an increase in the demand for offspring quality over quantity, 2) cultural imitation and 3) maladaptive traits. 4/19
Life History Theory, LHT, is the framework used to explore this theme in an evolutionary context. It is the inquiry into how organisms, in the face of constraints, allocate resources—time and energy—to maximise fitness. 5/19
According to LHT, energy devoted to reproduction is split into mating, parenting and nepotism, which is the caring for offspring of relatives. From this Voland (1998) identifies four basic trade-offs. 6/19
1) The trade-off between somatic investment, in one's own physical and social capital, and reproduction, 2) between own reproduction or that of your relatives 3) between mating and parenting, and 4) between the quantity and quality of offspring. 7/19
Evolution has equipped organisms to solve these trade-offs in an interchange with the environment, or ecology. Key point: in a modern context, shifts in external conditions are operating mainly on the first and fourth to produce a low-fertility environment. But how? 8/n
In economics, Gary S. Becker's work on this theme is seminal. Becker's key idea is that the income elasticity of offspring quality exceeds that of quantity. In a post-Malthusian world, rising income is used to increase the quality of offspring, education, not quantity. 9/19
In Becker's model, quantity and quality are substitutes, linked via relative prices. For a given number of quantity, one unit of quality becomes more expensive, and vice versa. In the 1980s, Becker & Barro extended this framework to take into account dynastic ambitions. 10/19
Unified growth theory, Galor (2011), offers a more general economic explanation for the fall in fertility during the DT, and relies on Beckers' intuition. It identifies an increase in the demand for, and return on, human capital as a key driver of falling fertility. 11/19
Galor (2011), and Guinanne (2011), also point to the rise in the opportunity costs of reproduction for women as an important driver of rapidly falling fertility during the DT, invoking one of the LHT trade-offs described in Voland (1998). 12/19
Kaplan (1996) offers an evolutionary/economic synthesis of the LHT framework and the Becker quantum effect. Kaplan shows, albeit in a small sample, how men have less children than predicted by their income, and how those that do don't seem to incur a fitness penalty. 13/19
Kaplan's hypothesis is contained in the following chart, linked to Becker. In a modern context, investment in human capital enjoys increasing returns to scale, and also accumulates across generations. This, in turn, generates a shift in the core quantity/quality trade-off 14/19
The Becker/Kaplan model raises a key question; can the standard quantum effect explain why fertility falls below replacement level and stays there in so many countries? The answer: perhaps, with some help. 15/19
What's the problem? The Becker/Kaplan model has an integer, or lower bound problem. It can't be consistent with no births, as you can't invest in something that isn't there. It probably can't, realistically, be consistent with fertility of one, or close to one, either. 16/19
But is population homeostasis a thing? Must fertility stabilise at replacement levels? Empirically, the answer is no. Theoretically? Voland (1998) categorically says no. So does Demeney (1997), but why then all the hand-wringing about maladaptive modern fertility trends? 17/19
Kaplan et al. (2002) argue that the interaction between the Kaplan/Becker quantum and the shift in women's opportunity cost of reproduction can, conceivably, drive fertility below replacement level, while still being consistent with fitness maximisation. 18/19
Kaplan et al. (2002) show that this interaction-effect can drive birth postponement, pushing fertility below replacement levels. This is exactly the tempo effect, which has been proposed as the key driver of the so-called second demographic transition, which I turn to next. 19/19