Research

The consensus view in popular media, official guidance from medical societies, and a body of literature based on small-sample historical populations, is that female infertility–“infecundity” in the language of formal demography–is mostly unchanging in the twenties, begins to decline in the early thirties, and then rapidly declines around age 35. The focal acceleration at age 35 has become enshrined in insurance coverage determinations and medical practice, with well-documented impacts on individual decision-making and healthcare access and use, including for pregnant women and women seeking fertility treatments. In this paper, we show that the consensus view is incorrect. Using a dataset of 2.8 million women aged 15 to 49 drawn from nationally representative samples of 62 countries, we generate precise, non-parametric estimates of fecundity decline with age. We find that there is no trend-break or acceleration in the mid thirties, and further that fecundity decline is steep through the late twenties (approximately as rapid as in the thirties). We show that the incorrect conclusions that have propagated in this area are due to an error in the way age specific-fecundity rates are typically calculated, creating a non-linear bias that introduces an artificial concavity in the age-fecundity relationship in the most widely-cited sources.

South India has low actual fertility, low reported ideal fertility, and — for the youngest cohorts — ideal fertility above actual fertility. Internationally, none of that is uncommon. What is uncommon is that women’s employment does not appear to be an important factor. We document new macro- and micro-level facts using India’s Demographic and Health Surveys. In macro international comparison, we see the unusual unimportance of employment in India’s unique combination of low fertility and low women’s labor force participation. In 2016, no country had female labor force participation below 40% and TFR below 2; 18 Indian states did. At the micro level within India, we see in Kitagawa decompositions that whether a woman works in the paid labor force and how explains little of large changes and differences in survey-reported ideal fertility. These facts pose a puzzle to theories that associate low fertility with women’s employment and its associated economic constraints.

“Future people count. There could be a lot of them. We can make their lives go better.” This is Will MacAskill’s elegant and compelling introduction to longtermism for a popular audience in What We Owe the Future . It is the starting point of an argument for prioritizing the wellbeing of the near-endless stream of future people. Or, more specifically, people who may exist if humanity can evade the nearer term existential risks that threaten it. In this chapter, we consider an important other possibility: There might not be a lot of them, after all. The entire population science community predicts the global population to begin shrinking within the lives of children born today. Once this decline begins, it may happen fast. The goal of this chapter is to bring facts from population science and population economics into dialogue with the community of longtermists who are thinking about wellbeing into the far future. To eventually achieve a flourishing far future, it is valuable that over the coming few centuries a complex global economy endures and the number of people does not become small enough to be highly vulnerable to extinction from a threat that a larger population could sustain. We review population projections and other social scientific facts that show that fertility rates that are normal in much of the world today would cause population decline that is faster and to lower levels than is commonly understood, threatening the long term future.

Human activity generates carbon emissions, leading many to conclude that declining fertility today could significantly reduce carbon emissions and the associated harms to future generations. We show that this conclusion is incorrect because it misunderstands population momentum, which ensures that even instant changes to fertility rates today have only small impacts on population size in the near-term, while emissions intensity remains highest. Further, prior assessments fail to account for countervailing benefits of a larger population: first, that larger population sizes have been a key contributor to economic growth; and second, that a retiree-heavy age structure stresses economic resources. Modifying a leading integrated climate-economy model, we show that a larger world population is projected to raise average future living standards, accounting for climate impacts.

The relationship between human population sizes and economic well-being has been long debated. We revisit this question in a modern, forward-looking setting using a semi-endogenous growth model with natural resources: people innovate, but dilute fixed resources. Conditional on a stable population, the model produces an analytical steady-state with a parsimonious sufficient statistic for the (local) relationship between long-run per capita income and population sizes. Calibration suggests a positive relationship between steady state population and GDP per capita at current levels. Furthermore, trends in natural resource shares provide evidence that humanity is not yet near the income-maximizing population level.

According to Longtermism, our acts’ expected influence on the value of the world is mainly determined by their effects in the far future. Given additive axiologies, such as total utilitarianism, there is a straightforward argument for Longtermism due to the enormous number of people that may exist in the future. This argument, however, does not work on person-affecting views. In this paper, we will argue that these views may, in fact, also lead to Longtermism. The reason they may do so is that Prudential Longtermism may be true. Prudential Longtermism holds for you if and only if our acts’ overall influence on your expected well-being is mainly determined by their effects in the far future. We argue that (due to a combination of anti-ageing, cryonics, uploading, and biological uploading) there could be an enormous amount of prudential value for you in the far future. This potential value may be so large that it dominates your overall expectation of lifetime well-being.

Utilitarianism is the most prominent family of social welfare functions. We present three new axiomatic characterizations of utilitarian (that is, additively separable) social welfare functions in a setting where there is risk over both population size and the welfares of individuals. First, we show that, given uncontroversial basic axioms, Blackorby et al.’s (1998) Expected Critical-Level Generalized Utilitarianism (ECLGU) is equivalent to a new axiom holding that it is better to allocate higher utility-conditional-on-existence to possible people who have a higher probability of existence. The other two novel characterizations extend classic axiomatizations of utilitarianism from settings with either social risk or variable-population, considered alone. By considering both social risk and variable population together, we clarify the fundamental normative considerations underlying utilitarian policy evaluation.

A critical open question at the intersection of climate change and demography is the relationship between extreme climatic conditions and human fertility. In this paper, we study how temperature and humidity exposure affects human fertility in the developing world. We combine 142 rounds of Demographic and Health Survey datasets from low and middle income countries around the globe to create the most complete catalogue of fertility patterns linked to weather data to date. Importantly, our analysis separates the direct impacts of weather from other place-specific seasonal factors that may influence fertility. We find that exposure to extreme heat—relative to a village or urban area’s typical seasonal temperature profile—lowers birth rates nine months later. We find that the rebound in fertility in subsequent months is incomplete, suggesting the fertility declines due to extreme weather are not completely reversed with later additional childbearing. These results have significant implications for climate change, both in describing the potential fertility consequences of climate change and because optimal climate policy depends crucially on the expected size of future generations.

Animal agriculture encompasses global markets with large externalities from animal welfare and greenhouse gas emissions. We formally study these social costs by embedding an animal inclusive social welfare function into a climate-economy model that includes an agri- cultural sector. The total external costs are found to be large under the baseline parameterization. These results are driven by animal welfare costs, which themselves are due to an assumption that animal lives are worse than nonexistence. Though untestable—and perhaps controversial—we find support for this qualitative assumption and demonstrate that our results are robust to a wide range of its quantitative interpretations. Surprisingly, the environmental costs play a comparatively small role, even in sensitivity analyses that depart substantially from our baseline case. For the model to find that beef, a climate-intensive product, has a larger total externality than poultry, an animal-intensive product, we must simultaneously reduce the animal welfare externality to 1% of its baseline level and increase climate damages roughly 35-fold. Correspondingly, the model implies both that the animal agriculture sector is much larger than its optimal level and that considerations across products ought to be dominated by animal welfare, rather than climate, effects.

According to Ex-Post Average Utilitarianism, prospect X is at least as good as prospect Y if and only if the expected average well-being is at least as great in X as in Y. This approach has the advantage (relative to the ex-ante approach of taking the average of individuals’ expectations) of not needing well-defined expectations of well-being for contingent individuals—individuals who exist in some but not all states of nature. It also has the advantage (relative to any other kind of average utilitarianism) of maximizing an expectation, which means it satisfies expected utility theory for general betterness. Nevertheless, Ex-Post Average Utilitarianism, we show, can oppose the interests of all affected individuals. Moreover, we can show this without assuming any well-defined expectations of well-being for contingent individuals.

In the literature on population ethics, economists typically assume a complete social ordering of variable-population welfare vectors. Among philosophers, however, completeness for variable-population (rather than fixed-population) cases is controversial. We ask how a utilitarian should respond to this controversy and demonstrate possibilities for weakening the assumption of variable-population completeness. Key to our result is the classic axiom of Existence Independence, which makes no unconditional variable-population comparisons and which we propose should be a core commitment of utilitarians, even those who are unsure of variable-population completeness. Our main result shows that completeness of a variable-population social ordering can be derived as a consequence of: Existence Independence, completeness for fixed-population cases, further uncontroversial axioms, and the minimum existence of variable-population comparability in at least one case. The upshot is that a utilitarian faces a choice between a fully complete variable-population social ordering or unlimited incomparability for every variable-population case. A consequence of our result is that we can weaken Blackorby, Bossert, and Donaldson’s (2005) foundational characterization of critical-level generalized utilitarianism to assume only fixed-population, rather than variable-population completeness.

All leading long-term global population projections agree on continuing fertility decline, resulting in a rate of population size growth that will continue to decline towards zero and would eventually turn negative. However, a literature inspired by mathematical biology has suggested that because fertility is heritable (i.e., higher-fertility parents tend to have higher-fertility children) and heterogeneous within a population, long-term population growth must eventually be positive. In this research note, we show that heritable fertility is not sufficient for positive long-term population growth, for empirical and theoretical reasons. First, empirically, even higher-fertility sub-populations show declining fertility rates which may eventually be below replacement (and in some populations already are). Second, in a simple Markov model, because heritability is imperfect, the combination of heritability and fertility rates may be quantitatively insufficient: it may be that higher-fertility parents nevertheless produce too few children who retain higher-fertility preferences. These results underscore the importance both of understanding the possible consequences of long-term fertility decline and depopulation and of the causal importance of culture and choice in human populations.

Derek Parfit argues that fission is prudentially better for you than ordinary death. But is having more fission products with good lives prudentially better for you than having just one? In this paper, we argue that it is. We argue that, if your brain is split and the halves are transplanted into two recipients (who both have good lives), then it is prudentially better for you if both transplants succeed than if only one of them does (other things being equal). This upshot rules out, among other things, that the prudential value of standing in the relation that matters in survival to multiple people is equal to their average well-being.

Tools are needed to benchmark carbon emissions and pledges against criteria of equity and fairness. However, standard economic approaches, which use a transparent optimization framework, ignore equity. Models that do include equity benchmarks exist, but often use opaque methodologies. Here we propose a utilitarian benchmark computed in a transparent optimization framework, which, could usefully inform the equity benchmark debate. Implementing the utilitarian benchmark, which we see as ethically minimal and conceptually parsimonious, in two leading climate-economy models allows calculation of the optimal allocation of future emissions. We compare this optimum with historical emissions and initial Nationally Determined Contributions. Compared with cost-minimization, utilitarian optimization features better outcomes for human development, equity, and the climate. Peak temperature is lower under utilitarianism because it reduces the human development cost of global mitigation. Utilitarianism, therefore, is a promising inclusion to a set of benchmarks for future explorations of climate equity.

What are the prospects for using population policy as tool to reduce carbon emissions? In this paper, we review evidence from population science, in order to inform debates in population ethics that, so far, have largely taken place within the academic philosophy literature. In particular, we ask whether fertility policy is likely to have a large effect on carbon emissions, and therefore on temperature change. Our answer is no. Prospects for a policy of fertility-reduction-as-climate mitigation are limited by population momentum, a demographic factor that limits possible variation in the size of the population, even if fertility rates change very quickly. In particular, a hypothetical policy that instantaneously changed fertility and mortality rates to replacement levels would nevertheless result in a population of over 9 billion people in 2060. We use a leading climate-economy model to project the consequence of such a hypothetical policy for climate change. As a standalone mitigation policy, such a hypothetical change in the size of the future population – much too large to be implementable by any foreseeable government program – would reduce peak temperature change only to 6.4°C, relative to 7.1°C under the most likely population path. Therefore, fertility reduction is unlikely to be an adequate core approach to climate mitigation.