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.

Grill (2023) proposes, as a candidate population axiology, the Sum of Averages View (SAV). Grill advances SAV because it escapes the Egyptology objection to average utilitarianism. But SAV escapes only the most literal understanding of the Egyptology objection. The challenges of non-separability immediately reappear when any unaffected group of people feels intuitively irrelevant for any reason other than their time of birth. Moreover, SAV has a decisive drawback not shared with either average or total utilitarianism: it can evaluate an outcome in which every individual is worse off as better overall, even when exactly the same people exist in both outcomes. These problems, we argue, afflict not only Grill’s particular view but any view that uses a sum of subpopulation averages, apart from the limiting cases of average and total utilitarianism.

Leading models imply a positive relationship between population growth and per capita economic growth—a larger labor force produces more non-rival ideas, which propel per capita productivity improvements. This paper demonstrates that this channel does not imply that faster population growth leaves individuals better off. The reason is straightforward: Faster population growth increases the arrival rate of both people and ideas, potentially leaving each life ex-ante unchanged. In ideas-based growth models, the population size governs the speed of history; the same events occur, they just occur earlier if the per-period population is larger. This reframing leads to two related insights. First, the distinction between processes that evolve per unit of time vs. per human life are crucial for assessing the effects of population growth. Second, for processes that evolve per human life to influence per capita outcomes, there must be a change in the number of people that ever live, not just the number who live in a given period. This makes assumptions about human extinction particularly important, despite the relatively limited attention (if any) typically paid to how models end.

The United States Supreme Court’s ruling in Dobbs v. Jackson Women’s Health Organization has made understanding the impact of abortion laws increasingly important and timely. We investigate recent claims by policymakers that abortion restrictions increase birth rates; we also evaluate consequences for human capital and women’s welfare. We motivate our theoretical contribution by presenting some simple empirical analysis of cross-country associations. These provide no evidence of a significant association between abortion legality and birth rates. Our main contribution is an applied economic theory model. Contrary to some policy claims, but in line with stylized empirical facts, abortion bans can lower equilibrium fertility: An abortion ban might cause women to have more unintended births at young ages, but this could reduce their accumulation of capabilities that would prepare them to have a larger family later. We solve a 2-period version of the model, and simulate it and a 3-period version. Our model shows that the sign of the effect on lifetime fertility depends on whether the increase in fertility due directly to unintended births is outweighed by the effect on subsequent fertility choices. But either way, abortion restrictions are likely to reduce human capital and harm women’s welfare.

Famous problems in variable-population welfare economics have led some to suggest that social welfare comparisons over such populations may be incomplete. In the theory of rational choice with incomplete preferences, attention has recently centered on the Expected Multi-Utility framework, which permits incompleteness but preserves vNM independence and can be derived from weak, attractive axioms. Here, we apply this framework to variable-population welfare economics. We show that Expected Multi-Utility for social preferences, combined with a stochastic ex-ante-Pareto-type axiom, characterizes Expected Critical-Set Generalized Utilitarianism, in the presence of basic axioms. The further addition of Negative Dominance, an axiom recently introduced to the philosophy literature, yields a characterization of Expected Critical-Level Generalized Utilitarianism.

We provide a general argument against value incomparability, based on a new style of impossibility result. In particular, we show that, against plausible background assumptions, value incomparability creates an incompatibility between two very plausible principles for ranking lotteries: a weak “negative dominance” principle (to the effect that Lottery 1 can be better than Lottery 2 only if some possible outcome of Lottery 1 is better than some possible outcome of Lottery 2) and a weak form of ex ante Pareto (to the effect that, if Lottery 1 gives an unambiguously better prospect to some individuals than Lottery 2, and equally good prospects to everyone else, then Lottery 1 is better than Lottery 2). After spelling out our results, and the arguments based on them, we consider which principle the proponent of incomparability ought to reject.

A smaller human population would produce less carbon emissions, other things equal. This fact has led to the view that an important benefit of the ongoing, global decline in fertility will be reductions in long-run temperatures. Here we assess the magnitude and economic significance of this relationship. We find that it is quantitatively small and, therefore, insignificant relative to other well-documented effects of population growth, including non-rival innovation. This conclusion follows from a key fact of timing: Population sizes would respond to any growth rate changes begun today with a many-decades lag, by which point per capita emissions are projected to have significantly declined. Therefore, the additional warming from any plausibly-sized change in population growth would be small when compared against the resulting long-run productivity impacts. Moreover, even the sign of the population-warming relationship is ambiguous when we account for the possibility of aggregate net-negative emissions.

The expectation of a sum of utilities is a core criterion for evaluating polices and social welfare under variable population and social risk. Our contribution is to show that a previously unrecognized combination of weak assumptions yields general versions of this criterion, both in fixed-population and in variable-population settings. We show that two dimensions of weak dominance (over risk and individuals) characterize a social welfare function with two dimensions of additive separability. So social expected utility emerges merely from social statewise dominance (given other axioms). Moreover, additive utilitarianism, in the variable-population setting, arises from a new, weak form of individual stochastic dominance with two attractive properties: It only applies to lives certain to exist (so It does not compare life against non-existence), and It avoids prominent egalitarian objections to utilitarianism by only applying if certain correlations are preserved. Our result provides a foundation for evaluating climate change, growth, and depopulation.

The relationship between human population sizes and per capita income has been long debated. Two competing forces feature prominently in these discussions. On the one hand, a larger population means that limited natural resources must be shared among more people. On the other hand, more people means more innovation and faster technological progress, other things equal. We study a model that features both of these channels. A calibration suggests that, in the long-run, the relationship between population and income per-capita is positive.

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.

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.

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. Relative to the ex-ante approach of taking the average of peoples’ expectations, this ex-post approach has the advantage of not needing well-defined expectations of well-being for contingent people—people who exist in some but not all states of nature. Nevertheless, we show that Ex-Post Average Utilitarianism can oppose the interests of all affected people. Moreover, we show this without relying on any comparisons of expectations of well-being for contingent people.

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.

Pritchett (1994) influentially examined the relationship between survey-reported ideal fertility and period total fertility rates. Pritchett’s analyses found that country-period by country-period differences in fertility preferences could statistically account for the majority of the variation in period total fertility rates. This result was impactful because it suggested that preferences, rather than constraints such as contraceptive access, explain fertility outcomes. However, because Pritchett’s analysis documented a correlation between two period measures, it cannot say whether a cohort of women achieves their fertility preferences, on average, over a life course. In the three decades since Pritchett’s paper, a longer span of repeated cross-section data has been collected that allows us to revisit this question with data that tracks cohorts of women as they age. In this paper, we update this evidence by examining the relationship between ideal fertility in early adulthood and completed fertility for the same cohort of women in later adulthood. We find that the prior result replicates: The relationship between fertility preferences and completed cohort fertility is, if anything, even stronger in our data.

According to Completeness, at least one of two prospects is at least as preferred as the other. I present a new money-pump argument that Completeness is a requirement of rationality. In comparison with earlier money-pump arguments for Completeness, the new argument relies on a form of unidimensional stochastic dominance and the behavioural assumption that agents pick in a probabilistic manner when no option is optimal. Moreover, unlike some of the previous arguments, the new argument is based on a forcing money pump, that is, an exploitation scheme where the agent is rationally required at each step to go along with the scheme

The size of the human population is projected to peak in the 21st century. But quantitative projections past 2100 are rare, and none quantify the possibility of a rebound from low fertility to replacement-level fertility. Moreover, the most recent long-term deterministic projections were published a decade ago; since then there has been further global fertility decline. Here we provide updated long-term cohort-component population projections and extend the set of scenarios in the literature to include scenarios in which future fertility (a) stays below replacement or (b) recovers and increases. We also characterize old-age dependency ratios. We show that any stable, long-run size of the world population would persistently depend on when an increase towards replacement fertility begins. Without such an increase, the 400-year span when more than 2 billion people were alive would be a brief spike in history. Indeed, four-fifths of all births—past, present, and future—would have already happened.

The climate-economy literature has documented adverse effects of extreme temperatures on wellbeing through mechanisms such as mortality, productivity, and conflict. Impacts due simply to discomfort are less well understood. This paper investigates individuals’ valuations of weather using a revealed preference approach. We first quantify the decline in attendance at Major League Baseball games on hot and cold days. Leveraging this finding coupled with the historically-informed assumption of a horizontal supply curve, we infer a monetized estimate of the disutility of extreme temperatures. We estimate a $1.53 utility loss per hour of exposure to high temperatures, implying non-trivial aggregate welfare effects.

There is growing international consensus that animal welfare is a crucial consideration in policy analysis, affecting domains ranging from food systems to biomedical research. Concern for animal welfare also features in many government regulations, certification programs, and institutional ethics codes across the globe and is central to many philanthropic and values-based investment decisions. However, although there are well-developed quantitative tools for incorporating human welfare into policy analysis, comparable tools for animal welfare are in their earliest stages. Without them, it is impossible to assess the net welfare impacts of a policy on humans and nonhumans alike on a common scale, which is crucial for making informed and transparent trade-offs (1). In practice, then, animal welfare is often ignored. Given that animal welfare matters in many cases, there is an urgent need for best-practice methods for integrating animal welfare into decision analyses.

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.

India has reached low fertility by mechanisms outside the traditional indicators of fertility decline. In contrast to countries that have achieved low fertility through delayed age at first birth, women in India have continued to enter unions and bear children early, lowered their age at last birth, and increasingly ended their fertility via sterilization following the birth of two children. Evidence from India reveals an alternative pathway to low fertility, highlighting the limitations of traditional socioeconomic indicators for explaining fertility decline.

Is the overall value of a world just the sum of values contributed by each value-bearing entity in that world? Additively separable axiologies (like total utilitarianism, prioritarianism, and critical level views) say ‘yes’, but non-additive axiologies (like average utilitarianism, rank-discounted utilitarianism, and variable value views) say ‘no’. This distinction is practically important: additive axiologies support ‘arguments from astronomical scale’ which suggest (among other things) that it is overwhelmingly important for humanity to avoid premature extinction and ensure the existence of a large future population, while non-additive axiologies need not. We show, however, that when there is a large enough ‘background population’ unaffected by our choices, a wide range of non-additive axiologies converge in their implications with some additive axiology — for instance, average utilitarianism converges to critical-level utilitarianism and various egalitarian theories converge to prioritiarianism. We further argue that real-world background populations may be large enough to make these limit results practically significant. This means that arguments from astronomical scale, and other arguments in practical ethics that seem to presuppose additive separability, may be truth-preserving in practice whether or not we accept additive separability as a basic axiological principle.

The time has come to include the wellbeing of animals in cost–benefit evaluations that inform agricultural policy. By doing so, we would account for those with the most to gain — or lose — from our choices.

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.

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.

The premise that a woman’s social status has intergenerational effects on her children’s health has featured prominently in population science research and in development policy. This study focuses on an important case in which social hierarchy has such an effect. In joint patrilocal households in rural India, women married to the younger brother are assigned lower social rank than women married to the older brother in the same household. Almost 8% of rural Indian children under 5 years old—more than 6 million children—live in such households. We show that children of lower-ranking mothers are less likely to survive and have worse health outcomes, reflected in higher neonatal mortality and shorter height, compared with children of higher-ranking mothers in the same household. That the variation in mothers’ social status that we study is not subject to reporting bias is an advantage relative to studies using self-reported measures. We present evidence that one mechanism for this effect is maternal nutrition: although they are not shorter, lower-ranking mothers weigh less than higher-ranking mothers. These results suggest that programs that merely make transfers to households without attention to intrahousehold distribution may not improve child outcomes.

Nearly all large policy decisions influence not only the quality of life for existing individuals but also the number—and even identities—of yet-to-exist individuals. Accounting for these effects in a policy evaluation framework requires taking difficult stances on concepts such as the value of existence. These issues are at the heart of a literature that sits between welfare economics and philosophical population ethics. Despite the inherent challenges of these questions, this literature has produced theoretical insights and subsequent progress on variable-population welfare criteria. A surprisingly bounded set of coherent alternatives exists for practitioners once a set of uncontroversial axioms is adopted from the better-studied welfare criteria for cases where populations are assumed to be fixed. Although consensus has not yet been reached among these remaining alternatives, their recommendations often agree. The space has been sufficiently restricted and well explored that applications of the theoretical insights are possible and underway in earnest.

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.

The Repugnant Conclusion served an important purpose in catalyzing and inspiring the pioneering stage of population ethics research. We believe, however, that the Repugnant Conclusion now receives too much focus. Avoiding the Repugnant Conclusion should no longer be the central goal driving population ethics research, despite its importance to the fundamental accomplishments of the existing literature.

Existing estimates of optimal climate policy ignore the possibility that carbon tax revenues could be used in a progressive way; model results therefore typically imply that near-term climate action comes at some cost to the poor. Using the Nested Inequalities Climate Economy (NICE) model, we show that an equal per capita refund of carbon tax revenues implies that achieving a 2 °C target can pay large and immediate dividends for improving well-being, reducing inequality and alleviating poverty. In an optimal policy calculation that weighs the benefits against the costs of mitigation, the recommended policy is characterized by aggressive near-term climate action followed by a slower climb towards full decarbonization; this pattern—which is driven by a carbon revenue Laffer curve—prevents runaway warming while also preserving tax revenues for redistribution. Accounting for these dynamics corrects a long-standing bias against strong immediate climate action in the optimal policy literature.

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.