Just under a century ago was the first time that as many as 2 billion people were ever alive at the same time. Today, there are nearly 8 billion of us. And our mere count is not the most important way that our lives are different from our great-grandparents’. Many of our ancestors would have thought it impossible that we could be at once so many and so rich.

More of us can read and go to school than ever before. Contraception and changing norms have given women more control over their own fertility than at any prior point in history. Our large, prosperous global population is also emitting more carbon than ever before, with environmental consequences that will eventually change where and how we can live. The greatest change may be yet to come. Because fertility rates are declining around the world, the size of the size of the human population will begin to shrink within the lifetime of children born today. This has never happened before, and once population decline begins, it may never stop.

We don’t know how the future will be better than today and how it will be worse. In part, this is because we don’t know which changes should count as good. Our human population has never quite agreed on how to value its own size.

Is it good or bad if more of us are alive? To answer an easier question, it is certainly good if parents, and especially mothers, get to choose how many children they have and when, so babies are born to parents who want them. Another easy observation is that more people cause more pollution and crowding. But more people also create more science, art, and friendship. And if their lives are good — if they are fun, rewarding, valuable, joyful — then more good lives means more goodness.

Consider an imaginary case of two possible populations, one larger and one smaller. Everyone in both populations is just as happy, and nothing is lost in the expansion of population size. Does the fact that more good lives have more goodness mean that the larger population is better? Many researchers who have considered this hypothetical question think so.

But if adding more good lives is indeed better, then we face a harder question. When can a larger population be a better population, even if people are a little worse-off? Parents know that creating new people brings its joys, but also brings its sacrifices. What about the sacrifices and tradeoffs that we might face as a world population? How prepared should we be to sacrifice some average wellbeing so that more people can live to share in the experience?

Here, the experts do not agree. “Population ethics” is the name for the field at the intersection of economics and philosophy that asks these sorts of questions. Experts are paid to disagree with one another, so it is no big surprise that the professors who work on these questions have not settled matters. But in this case there has been a special way that deadlock has endured: something called the Repugnant Conclusion.

About four decades ago, under the leadership of a philosopher named Derek Parfit, population ethics had an arresting realization. The plausible ways of evaluating and ranking the possible futures for humanity, Parfit worried, all lead to a conclusion that he did not like.

Here is the basic idea of the Repugnant Conclusion. Picture some people who are very well-off, living great lives. Now imagine if the whole human population were full of only people like them. No bad lives. Impossible, because inequality is real, but great, right? Call that the “small population.” (The whole human population may not seem small, but wait for it.)

Now picture a life which is better than never having been born at all, but is only a little better than never having been born at all, and surely worse than one of the great lives in the small population. Imagine that there could be an enormous population where very many people get to live, but they only live these worse lives. Could the enormous population ever be better than the small population? The Repugnant Conclusion says yes:

Imagine any small population that you can conceive with great lives. Whatever you choose, there is some hypothetical version of the enormous population that would be better. That is, one could conceive a large enough population full only of the worse lives, so that any correct ethical framework would rank this large population as better than the small population. Better, here, means not just better in my opinion or yours, but truly better, all things considered.

Many of the leading theories of population ethics have the bug, feature, or by-product of telling us that the Repugnant Conclusion is true.

The exact formulation of the Repugnant Conclusion varies between the precisely-chosen words of the philosophers and the detailed math of the economists. But everybody agrees it is a conclusion only for imaginary cases. It does not make any recommendations for real-world policy. (If you are worried that this story seems to have strayed from pressing questions about climate change, reproductive health, pension plans, and other important matters of policy, we are too.)

The story of the Repugnant Conclusion could have been short. In the late 1980s, an economist named Yew-Kwang Ng proved a mathematical fact: any way of ranking the overall goodness of all imaginable populations would imply the Repugnant Conclusion (in some hypothetical case) provided that the method followed the two rules. Rule 1 is that inequality is not better than equality. Rule 2 is that adding an extra person whose life is not bad does not in and of itself make the world worse, if that person’s existence has no negative effects. Ng showed that the Repugnant Conclusion is as undeniable as these rules are.

But Parfit labeled this inescapable conclusion “repugnant” and many philosophers agreed. This created a logjam in the literature. Population ethicists have focused on it ever since. The Stanford Encyclopedia of Philosophy explains that “the question as to how the Repugnant Conclusion should be dealt with and, more generally, what it shows about the nature of ethics has turned the conclusion into one of the cardinal challenges of modern ethics.”

And yet, more books and papers about the Repugnant Conclusion have not helped us understand what to do about climate change, or reproductive healthcare, or the education of girls in developing countries, or dwindling fertility rates in rich countries.

So, one by one, writers began to suggest that maybe the Repugnant Conclusion is not such a big deal after all. Perhaps, some suggested, whether and how to avoid it is not the most important question in population ethics. Perhaps, others speculated, we cannot really imagine enormous populations, so we just don’t have good reason to trust our initial reaction that the Repugnant Conclusion is a problem. Perhaps, some found, the more you think about the Repugnant Conclusion, the less the bits and pieces of it even make sense at all. If the Repugnant Conclusion turns out to be meaningless, it could hardly be a problem. And perhaps, some concluded, if enough arguments find that the Repugnant Conclusion is true (in hypothetical principle) then we should be open to believing it. That is what formal arguments are for.

After all, some said, it was never exactly spelled out why the Repugnant Conclusion was supposed to be so wrong in the first place. Parfit’s own explanation was that a world in which a few people had the best experiences in life had to be better than a world in which more people had only “lower pleasures.” Otherwise, the best things — the best symphonies, for example — wouldn’t exist. But this sort of elitism has no place in ethics. Parfit himself wrote elsewhere the we ought to give “priority” to people who are worse-off.

The requirement that population ethics must avoid the Repugnant Conclusion was what philosophers call an “intuition.” But is an intuition about trillions of lives unlike our own really something we can use to build the theory behind climate policy, education policy, and all of the economic and social issues that interact with population growth? A deeper explanation of why this conclusion is so uniquely repugnant turned out to be hard to find. Such an account would have to do more than explain away the formal arguments that the Repugnant Conclusion is inescapable.

Such an account would also have to explain why the Repugnant Conclusion is so special. How is it different from all of the other similar conclusions in population ethics and economics that make similar claims about large and small sets of people? One has to slice and dice the technical definitions to make the Repugnant Conclusion appear any different from other familiar puzzles: Should we help many people a little or help a few people a lot? So, every few years, somebody would write a paper suggesting that the Repugnant Conclusion is not really different or special, after all.

People began to suggest moving on. “It is time to retire the Repugnant Conclusion from population ethics,” Partha Dasgupta wrote in 2016. “Perhaps the best solution, on balance, is to revert to ‘total’ prioritarianism [an approach that adds a measure of goodness across everyone] and accept the Repugnant Conclusion,” Matt Adler proposed. Tyler Cowen, calling for faster technological growth for a better future, dismissed the Repugnant Conclusion as a constraint: “I say full steam ahead.”

A few weeks before COVID-19 shut down everything, some of us philosophers and economists gathered at the University of Texas in Austin. We met to share research about the interactions among climate change, population change, and economic policy. Over dinner, we confessed to one another that we did not think that figuring out what to do about population policies required avoiding the Repugnant Conclusion.

Together, we concluded that it is not necessary to find a way to avoid the Repugnant Conclusion before doing any further research on population ethics. It is not necessary to avoid the Repugnant Conclusion to have a good enough theory.

We asked around. We found 29 economists, philosophers, and demographers who had written about these issues and who agreed with us. Our collaborators included leaders of the field — like Yew-Kwang Ng of the 1980s math proof — as well as emerging thinkers — like Michal Masny, a Princeton student who recently won a prize for a paper showing that even Parfit’s own attempts to escape the Repugnant Conclusion end up at the same old Repugnant Conclusion, yet again. We came from different universities and different continents.

We also quickly realized that we came from different perspectives. So we wrote down as much agreement as we could. The first point of agreement is this: “avoiding the Repugnant Conclusion is not a necessary condition for a minimally adequate… approach to population ethics.” That does not claim that the Repugnant Conclusion is true (some of us think it is, some of us think it’s not). Instead, it suggests a reallocation of attention.

Because we are a group of 29 researchers, we disagreed about almost everything else. It is remarkable that we got as far as we did. We probably wouldn’t agree that this article (which only a few of us wrote) tells the right story. We do not even agree about why we agree about what we agree about. But the Repugnant Conclusion gets enough attention that it was worth writing down the fact that we agree.

This week, the journal Utilitas is publishing our multi-author statement of agreement (open access to the published paper at this link). Maybe pausing to note where we agree can become a habit among population ethicists, rather than the exception.

Our collaborative paper ends with the hope that population ethics will one day consider the puzzles of our times resolved and will turn to other things. “Many public decisions affect the world’s future population. Population ethics is therefore an essential foundation for making these decisions properly. It is not simply an academic exercise, and we should not let it be governed by undue attention to one consideration.” Population ethicists have learned from considering the Repugnant Conclusion. We can begin to apply what we have learned to other pressing questions.

Although Medium only recognizes one author, Dean Spears collaborated with Mark Budolfson, Johan Gustafsson, and Stéphane Zuber to write this essay and to lead the 29-author collaboration we describe.