We at the Center for Bioethics have been honored with an unexpected privilege. In the course of preparing his capstone project, Tim Furlan, PhD, assembled a suite of articles for the HMS Bioethics Journal, among them a contribution from Dan Callahan completed just before his death. Fittingly, the article reaches across generations and disciplines to address some of the larger questions engendered in bioethics and by the human condition. We refer our readers to The Hastings Center President and Center for Bioethics Fellowship Director Mildred Solomon’s In Memoriam about Dan—his life, achievements, and lasting impact. —Eds.
I am writing this paper on the 50th anniversary of the Hastings Center. Anniversaries can lead us to muse on the past or the future. As for that past, I am, at 89, a survivor of the “log cabin” days of bioethics. Bioethics is the natural child of scientific and medical progress, born and raised in the 1960s, and still growing. My musings, and indeed, argument for the future of bioethics is based on the necessity of finding the best balance between the benefits and harms of progress. I chose progress for a simple reason. Progress has given us most of the ethical and social benefits of modern medicine, alongside the most aggravating ethical challenges. I conclude with a proposal for the next 50 years of bioethics.
I want to examine those challenges in four steps. The first is a short reminder of the rich history of the concept of progress. That is followed, second, by a survey of five case examples of current ethical dilemmas: population growth and aging societies, health care, genetics, end-of-life care. Third, I single out some different ways of thinking about progress, which I believe are novel. I conclude with an examination of what I call finitude progress and end-of-life care—more than a theoretical matter for someone my age.
The History of Progress
There is a long and interesting literature on the concept of progress, going back, some would argue, to the Greeks, but primarily centered on the Enlightenment of the 17th and 18th centuries and a foundational component of American philosophy.  The dictionary definition catches well the idea: “forward movement toward a refined, improved, or otherwise desired state,” and “development toward improved or more advanced conditions.” , ,  Progress has been and can be used in many contexts. Some view progress as economic growth.  However, economic growth is in large part a measure of the development, and spread of access to, the fruits of scientific advances, and GDP doesn’t reflect cultural changes. , ,  Thus, the most common context now is that of scientific  and cultural  movement, with the benefits of the advance of scientific knowledge taken to be its most important feature. ,  Medicine and health care have been its among its greatest beneficiaries: the saving of life and the reduction of illness. 
There are, however, some features of progress that are neglected. The connotations of progress are invariably positive: new scientific advances are usually embraced. As I will shortly contend, however, what begins as progress, promising desirable benefits, can over time turn burdensome or harmful. A peculiar feature in the history of the concept of progress is that nothing is ever labeled as “bad” or “harmful” progress. There is discussion and debate about the rise and decline of progress enthusiasm over time, lamentations about temporary declines as a sign of cultural failure, and complaints by some that enthusiasm for progress has led to unfortunate declines of older ideas and values. The poet T.S. Eliot became famous in part for his defense of them.  There has also been a long debate among historians whether ancient Christianity supported the idea of progress.  By and large, however, progress is now understood as a secular value resting on science, reason, and a deep belief in the value of constant change and improvement of the human condition. ,  Some indeed have called it a kind of secular religion, a faith that progress can save us from dangers and harms and give us a better life. I would characterize it as a transcendent ideal, a high value, sometimes a dangerous, and well-practiced seductress.
1. Population growth and Aging Societies.
I put the two together for analysis because while separate, they are also entwined. End-of-Life care touches our individual lives at its most primeval level. Population growth and societal aging bear on our common life. What are the social consequences of procreation and aging trends?
In 1969, when I was just beginning to put the Hastings Center together, I was asked by the Population Council in New York to undertake ethics research on population growth and family planning. In 1789, Thomas Malthus’ An Essay on the Principle of Population had called attention to the threat of excessive population growth, particularly on food availability, but it was not until the 1960s and effective contraception that the United Nation and private organizations worked to reduce family size.  That effort had limited success. In the mid-1980s, the UN shifted from the provision of contraceptives as its main strategy to stress the education of women as the most effective way to lower birthrates. 
With its limitation of parenthood to one child in that era, China stood out. In most developed countries, birthrates dropped below the 2.1 child-per-woman level necessary for population stability, and many countries below 1.5 child-per-woman.  For the most part, only the sub-Saharan countries have 3.5 and higher fertility rates.  However, by the 1970s, many of the developed countries, notably China, became alarmed at low birthrates; a growing imbalance between young and old meant relatively fewer young people to care for more old people. All the while during the decades of the 1960s to the present, average life expectancy was increasing, and notably so among the old.
A dose of data is pertinent here. In 2017, the United Nations projected global population growth will rise from 7.6 billion in 2015 to 9.7 billion in 2050 and then to 11.2 billion in 2100.  Global average life expectancy is projected to rise from 71 years to 77 years in 2040-50. The number of people aged 60 and over, 962 billion in 2017, will rise to 2.1 billion in 2050 and then to 3.1 billion in 2100.  Among the elderly, those over 80 will increase from 125 billion in 2015 to 944 billion in 2100. The growing imbalance between young and old is already increasing family care needs for the frail old.
The population pyramid, above, visualizes the combined effects of low birth mortality and longevity on population distribution by age; median age in 1950 was 23.6 years, and by 2050 will have increased to 36.1, placing a large percentage of the world's population, nearly one billion, at eighty years old or older.
2. Health Care
The American health care system has long been a source of pride and vexation, a political football. The pride comes from its research contributions and fine practice in many clinical areas. The vexation arises because the US health care system spends more money per capita to achieve worse health results than other developed countries.  As the 2019 political scene shows, the struggle over health care is shaping up to be a central political issue once again. Republicans are pushing to reduce or end the present Affordable Care Act (ACA), while Democrats are offering various universal care plans to improve the ACA or replace it with “Medicare for All.”
One way or the other, a key feature of our system is likely to remain, that of leaving much health care to the private sector. Progress is profitable and its purveyors have political clout. The high and always rising price of drugs stands out as a marker of the basic tension, deeply imbedded in our culture, between market values and government power. In other developed countries government can control drug prices, but not in our system.
Direct outlays for healthcare made by users as a percentage is shown by gradient above. Live visualization is available here.
Many of us in the U.S. have long envied the European universal care, with supposed guaranteed health care from cradle to grave. Universal care? Rarely noted is that those systems share with the U.S. copayments and deductible, ordinarily called out-of-pocket costs. Most are not as high as here, but the leader is Switzerland, a wealthy country but with the highest out–of–pocket costs. Some 20% of my social security coverage goes for my supplementary Medicare coverage. It is no secret, but not much noticed in the U.S., that many developed countries are worried about the future of their systems.  They reflect the same surging strain of elderly coming and the declining portion of young people to support them. 
The announcement in 1953 by James G. Watson and Francis H.C. Crick of their discovery of the double-helix structure of DNA was perhaps the greatest of the 20h century. If a bit immodestly, Crick said that “we have found the secret of life.”  It generated great hopes, among them pre-natal screening, disease cure, genetically engineered foods, and means of convicting or exonerating criminals.
Two scientific developments since then have brought ethical worries to the fore and important scientific responses. They have a remarkable similarity. The first arose in 1975 when geneticists working with what are called recombinant DNA. The concern of the scientists was that the research could cause biohazards, including cancer-causing infection’ In response to those concerns, the scientists declared an unprecedented moratorium on the recombinant research, and assigned a risk estimate for the different types of research.  There was remarkable consensus among the scientists, but with one notable exception: James D. Watson publicly scorned it as a threat to the freedom of scientific research. 
Paul Berg, a leader of the Asilomar scientific organizers, wrote in a 2008 retrospective article in Nature magazine with a memorably powerful defense of the moratorium: “The best way to respond to concerns created by emerging knowledge or early stage technologies is for scientists…to find common cause with the wider public about the best way to regulate as early as possible. Once scientists from corporations begin to dominate the research, it will simply be too late.” 
The other struggle is strikingly similar, On December 2, 2015 the National Academy of Sciences, Medicine, and Engineering released a statement by the International Summit on Human Gene Editing—focused on using a technique labeled as CRISPR—Cas9.  There were three recommendations. It supported the continuance of basic and preclinical research. It supported as well the editing of somatic cells, those not passed along to the next generation. The third recommendation was that the “use of gene editing for human gametes or embryos it is unacceptable” when “the cells of a resulting child will be passed on to subsequent generations as part of the human genome.” That standard was violated in 2018 by a Chinese researcher, He Jiankui and colleagues.  Hi research was carried out secretly violating Chinese guidelines as well as those recognized in the 2015 Summi.
4. End-of Life-Care
Death itself, as individual threat and human puzzle, is as old as human life. An ancient king, Gilgamesh, was in 3000 BC reputed to say, “I am afraid of death and roam over the desert.”  In the end he accepts death: “the gods have allotted death to mankind.” Far later, the historian Philippe Aries in his 1971 book The Hour of Our Death, wrote of death through the ages as a “tame” event, one that had a “familiar simplicity,” little altered by medical or social change. 
By the end of the 1960s, a turning point was evident. It was marked by an explosion of medical research, the passage of the Medicare and Medicaid programs, and medical progress on most fronts. An important consequence was that of longer lives and extended dying, the loss of a tame death. The advent of the hospice movement, initiated by a British nurse, Cicely Saunders, was a singular response to prolonged, difficult deaths.  It was accompanied at the time by other important features. One was the advent of a shift from physician paternalism to patient choice; the advent of the “living will.” The other was better, more sensitive treatment of patients by physicians.
If, for many thousands of patients, hospice care has brought a tame death, for many it has succeeded only in part. Care of the dying is still a problem, and perhaps always will be.  The combination of changing technologies, variation in physician skills, and patient/family disagreements seem perennial. The advent of spreading physician-assisted death (PAS) and euthanasia add an intensified feature to debates on dying. At least in the U.S. most patients who choose PAS have passed through hospice.
I have been impressed by the insight the sociologist Sharon R. Kaufman has brought to the care of the dying in her book Ordinary Medicine: Extraordinary Treatments, Longer Lives and Where To Draw The Lines.  She identifies the “hidden” forces in the health care system: “regardless of a patient’s age, most deaths have come to be seen as premature….Our romance with technology is fed by the American ethos that more is always better.” The net result is that a “chain of health care drivers” has control of the system. It “controls health system values and ethical choices.”
Darker red indicates greater length of life with disease burden. Between 1990 and 2016, average number of years lived with disability increased from just over ten years to just over 11 years. Live visualization available here.
Another group of researchers, often called transhumanists, seek to radically extend life expectancies. Its putative leader over the years has been Aubrey deGrey, leader of the SENS Research Foundation. He once proposed lifespans to 1500 years, but modified it to 1,000. A more moderate movement, led by S. Jay Olshansky, aims to improve aging, focused on the “compression of mortality,” that of reducing the burdens of aging.
Progress: Its Power and Future
I chose each of my five bioethics examples for a particular reason. They are the fruits of progress, usually unanticipated. For the purpose of this analysis I want to distinguish between different forms of progress, each of which raises a different kind and result of progress, and requires different responses.
I begin with what I call restorative progress. By that phrase I mean the phenomenon of progress developments that look full of promise, that begin well and seem to have a long life ahead, and then over time turn sour, creating the need for a new direction, a redirection of progress. What begins as a human benefit turns out to be harmful in the long run, eventually calling for a different form of progress to rescue us from that unforeseen harm. Excessive population growth is the result of progress towards healthier lives. In turn, that new-found health brought extended longevity and excessive population growth—and with it the problems of aging societies: two few young people to support too many longer-lived old people. What kind of progress will be needed to cope with this development to bring us back to a good balance?
By equity progress I mean the fair distributions of population benefits from medical and social progress, notably the availability of good health care, with the poor getting a fair share of rising GDPs. But rising GDPs are rarely equally beneficial, and all the more when market values are dominant.  The U.S. stands out among developed countries both for its high GDP and its health care arguments and erratic health care provision.
By prudential progress I have in mind the special dilemmas of genetics research, where the research itself may bring immediate benefits for some but at the risk of long lasting and irreversible harm later on for others. It may also apply to efforts to radically extend human life expectancies, but with a failure to match the enthusiasm of the effort with a careful, prudent effort to imagine what a life expectancy of 150 or 200 could be life for us as individual and collectively for society.  Prudential progress is pertinent as well for genetic research. This problem is conspicuous in the scientific debate over CRISPR-Casp5, with those concerned about the danger of future harm pitted against the danger of holding up research progress.
Finitude progress is the effort to extend life in the face of inevitable death from disease and other lethal events. Since we are as humans biologically organized to die, later if not sooner, the best that medicine can do is to keep our body going, normally by disease-preventive and medical technological strategies. Unfortunately, there is also what one might call a perversity: the cure of a lethal disease in a patient inevitably opens the door to death from some other cause. Your life saved from cancer makes it certain you will then die of heart disease or Alzheimer’s or something else that kills people. In the end, we all die, the victim of our finitude. Cancer surgeons are not likely to point this out to their patients. About half of the disability and loss of functioning experienced in older ages is the result of lethal diseases; the rest are conditions not linked to mortality trends. 
Between 1990 and 2016, dementia-related deaths have more than doubled worldwide.
The other form of finitude progress comes with care of the dying: how much and what kind of treatment to expend on the dying? Technological progress, with steady incremental advances, brings with it (as Sharon Kaufman’s research shows) the possibility and temptation not to give up, to nurture failing hopes, to try one more possibility, then still another: don’t give up hope. 
Dilemma progress refers to those forms of benefits that can be achieved only by harmful or risky means. I have in mind here the testing of new drugs or surgical procedures, but where there is more than one strategy to save a patient, and where the best may also be the most hazardous or painful.  And sometimes of course there are low probability last ditch treatments where the patient dies on the operating table.
What Do We Do?
I have laid out a daunting list of challenges. We live in a global world that now has ‘progress’ built into its culture. The cheerleaders for progress, and their Silicon Valley step children getting rich on “innovation,” can always find an audience. Researchers for disease cures will always find money. Can progress be tamed? Not easily. Each of the progress challenges I have cited admit of a solution, at least theoretically. Creative incentives to encourage increased procreation, now declining globally, can be pursued. More children now would mean more caretakers and providers later to support and care for the elderly.
It is a nice idea, but it does not work. Many European countries and China began in the late 1970s to develop pronatalist policies toward that end. By and large, they have been a striking failure everywhere,  most notably in China, which without success has failed to increase procreation to replace its earlier famous one-child limit.  In addition, and quite apart from concern about the elderly in more affluent countries, the UN and other international programs are working to reduce the persisting high birthrates in many poor countries, most notably in sub-Saharan Africa.  In more recent years, moreover, concern for global warming, and perhaps global warming itself, is resulting in low global birth rates,  particularly in tropical regions.  While there are many ideas under discussion about present and future care for the elderly in the face of declining younger populations, they amount mainly to tinkering with present policies, primarily later retirement age, reduced work demands for the elderly, and government support of caretakers.
How Long Should We Want to Live?
I end with that question because death is, for medicine, the ultimate evil: the essence of finitude. It is possible that if the desire to live was more modest, lives could be shorter in the future. I want to make the case that the aim of progress over the next 50 years should be to work toward the return of a “tame death.” By that I mean shorter lives and more rapid, peaceful deaths. In my parent’s generation, born in 1895-1905, death was most commonly in a person’s 60 and 70s, living into one’s 80s was unusual, and anyone over 90, much less 100, a rarity. So far as I can recall, it was still the era of the “tame death.” Weeping at funerals of the elderly was rare, and the wakes, as often as not, were occasions of hugs, kisses, and the exchange of fond memories.
Perhaps with those memories in mind, I put forth in my 1987 book Setting Limits: Medical Goals In An Aging Society, this question: “how long should I want to live?” I was then a callow youth of 47. I offered a three-part answer to what I called a “tolerable” death: “(a) one’s life accomplishments have on the whole been accomplished; (b) one’s moral obligations to those for whom one has obligations have been discharged; and (c) one’s death will not seem to others an offense to sense or sensibility, something to incite in others a sense of tragedy.”
By that last standard I have in mind not just the death of children and young adults, or those who die from mishap or violence, deaths that are anything but tame. The death of a loved one in old age, even if “tame,” is likely to always be sorrowful, but extended old age can involve as much loss as a shorter period of old age. I specified that an acceptable life span using my criteria would be about 80, but some public opinion surveys have found 90 to be the most desirable life span. Setting a goal, specifically about desired longevity, would be part of the drive for a tame death. Remarkably, a recent survey in Belgium found that 40% would limit government supported health care at age 85.  A recent book on four outstanding American presidents noted that all died quickly and before the age of 70 (Washington, Theodore and Franklin D. Roosevelt, and Lyndon Johnson) Their tame deaths contrast with that of Winston Churchill who died slowly and unpleasantly at age 90, well after his political career was over.
Let me briefly mention some of the benefits of this age-limited model in light of my earlier five bioethics examples studies. I don’t believe I would have missed much in my life had I died at 80. The elimination of my last decade or so would not have been a societal loss. If my tame model was in effect and generally acceptable, population expansion and aging growth would significantly decline, reducing the imbalance between generations. Health care costs would decline because there would be fewer numbers of patients, young and old, and the burden on caretakers lessened.
There would of course be a sharp decline in Alzheimer’s and other diseases of advanced aging. I am not sure what, if any, decline in genetic-influenced disease would occur (but that may display my lack of imagination). Finally, there could be a great difference in end-of-life care. High technology, last chance therapies, and care for the dying would ordinarily be available only to those 80-90 and under.
I can foresee some obstacles to even getting a discussion of a return to a tame death started. One will come from the large tribe of progress enthusiasts, led by Steven Pinker and the large cadres of innovation chasers and profit-driven medical corporation researchers. Progress pays. They will see my kind of talk as dangerous.
Another obstacle, resting in part on the others, is that it would be unethical to go this route, indeed to even speak of it in public, much less someone trying to put it into practice. But if my memory is correct, the tame deaths in my parent’s generation were not treated as evil. Historically, most of the present interest in increasing longevity came in the post WWII period, and the era of embracing progress that brought forth bioethics to cope with the ethical fallout that resulted from rapid technological advancements in medicine.
But I have long believed that bioethics is at its best when it takes on the ideas and social structures that shape our collective and individual lives. It is hard to find any value more potent than the idea of progress. To put that value at work to regain a tame death—restorative progress—would both honor and humble progress as traditionally understood. It would also help relieve pressure on the issues which make up the case studies discussed here, each suffering from the fruits of progress. That restorative progress goal may well require the next fifty years to achieve, but it would be worth the effort.
Daniel Callahan was, with Willard Gaylin, the cofounder of the Hastings Center in 1969 and served as its president for many years.
 Commager, Henry Steele. The empire of reason: how Europe imagined and America realized the Enlightenment (Garden City, NY: Anchor Press, Doubleday, 1978): xii, 40, 44, 77ff, 97.
 Collins English Dictionary. 7th ed. (Collins, 2015). Continually updated at https://www.collinsdictionary.com/dictionary/english/progress.
 Oxford English Dictionary. 3rd ed. (Oxford, UK: Oxford University Press, 2010). Continually updated at https://en.oxforddictionaries.com/definition/progress.
 Cambridge Dictionary. 4th ed. (Cambridge, UK: Cambridge University Press, 2013). Continually updated at https://dictionary.cambridge.org/dictionary/english/progress.
 Bishop, Matthew. "Beyond GDP." The Economist (London, UK) April 18, 2013. https://www.economist.com/feast-and-famine/2013/04/18/beyond-gdp.
 Insofar as progress is movement towards a “desired state,” studies show the link between economic growth and, for example, happiness, is quite weak. See:
Veenhoven, Ruut. Conditions of Happiness (Dordrecht, NL: Springer Netherlands, 1984): 152-154.
 See also: the "Easterlin Paradox"
Easterlin, Richard A. "Will Raising the Incomes of All Increase the Happiness of All?" Journal of Economic Behavior and Organization 27, no. 1 (1995): 35–48. https://doi.org/10.1016/0167-2681(95)00003-B.
 Nisbet, Robert A. History of the Idea of Progress (New York: Basic Books, 1980): Ch. 5.
 Moore, Thomas G. "Chapter 1: The meaning of progress" in On Progress: Its Reality, Desirability, and Destiny (Stanford University: Online, 1994): 1-4. https://web.stanford.edu/~moore/Chapter1.pdf.
 Compare: Woods, Erville Bartlett. "Progress as a sociological concept." American Journal of Sociology 12, no. 6 (1907): 779-821. https://www.jstor.org/stable/2762650.
with: Nisbet, Robert A. History of the Idea of Progress (New York: Basic Books, 1980): Ch. 5. nearly 75 years later.
 Knudsen, Lara M.. Reproductive Rights in a Global Context (Nashville, TN: Vanderbilt University Press, 2006): 1-4.
 United Nations. International conference on population and development, Cairo, Egypt. September 5-13, 1994. United Nations Population Information Network (POPIN). https://www.un.org/popin/icpd/infokit/infokit.eng/1overvw.html.
 United Nations Population Division. World Population Prospects: 2017 Revision. Fertility Rate, total (births per woman). The World Bank Data. https://data.worldbank.org/indicator/sp.dyn.tfrt.in?end=2017&most_recent_value_desc=false&start=1960.
 United Nations Population Division. "Key Findings and Advance Tables" in World Population Prospects: The 2017 revision. United Nations (2017): 1-2. https://population.un.org/wpp/Publications/Files/WPP2017_KeyFindings.pdf. Accessed July 29, 2019.
 Rapaport, Lisa. "U.S. health spending twice other countries’ with worse results." Reuters (London, UK) Mar 13, 2018. https://www.reuters.com/article/us-health-spending/u-s-health-spending-t....
 Paun, Carmen. "Europe’s health systems on life support." Politico (Arlington County, VA) October 7, 2016. https://www.politico.eu/article/europe-health-care-systems-on-life-suppo....
 National Institute of Health. "The Francis Crick papers: The discovery of the double helix, 1951-1953." National Library of Medicine. https://profiles.nlm.nih.gov/SC/Views/Exhibit/narrative/doublehelix.html
 Berg, Paul and Maxine Singer. "The recombinant DNA controversy: twenty years later." Proceedings of the National Academy of Sciences of the United States of America 92, no. 20 (1995): 9011-9013. https://doi.org/10.1073/pnas.92.20.9011.
Culliton BJ. "Recombinant DNA: Cambridge City Council votes moratorium." Science 193, no. 4250 (1976): 300-301. https://doi.org/10.1126/science.11643322
 Watson, James, Andrew Berry, and Kevin Davies. The Story of the Genetic Revolution (Toronto, CA: Alfred A. Knopf, 2017).
 Committee on Science, Technology, and Law; Policy and Global Affairs; National Academies of Sciences, Engineering, and Medicine. "International Summit on Human Gene Editing: A Global Discussion" edited by Steven Olson. Washington, D.C.: National Academies Press, December 1-3, 2015. https://www.nap.edu/read/21913/chapter/1
 Kluger, Rivkah Shärf. The Archetypal Significance of Gilgamesh: A Modern Ancient Hero (Einsiedeln, CH: Daimon Verlag, 2015) :Tabl. IX, col. i, lines 15, Heidel p. 64.
 Aries P. The Hour of Death: The Classic History of Western Attitudes Toward Death over the Last One Thousand Years. (Toronto, CA: Alfred A Knopf, 1981).
 Woo, Jennifer A., Guy Maytal, and Theodore A. Stern. "Clinical challenges to the delivery of end-of-life care." The Primary Care Companion to The Journal of Clinical Psychiatry 8, no. 6 (2006): 367-372. https://doi.org/10.4088/pcc.v08n0608.
 Kaufman, Sharon R. Ordinary Medicine: Extraordinary Treatments, Longer Lives, and Where to Draw the Line (Duram: Duke University Press, 2015).
 Samans, Richard. "A new way to measure economic growth and progress." World Economic Forum (Cologny, CH) January 22, 2018. https://www.weforum.org/agenda/2018/01/towards-a-new-measure-of-growth/.
 Andersen, Ross. "Radical life extension is already here, but we’re doing it wrong." The Atlantic (New York, NY) May 21, 2012. https://www.theatlantic.com/health/archive/2012/05/radical-life-extension-is-already-here-but-were-doing-it-wrong/257383/.
 Crimmins, Eileen M. "Americans live longer, not necessarily healthier, lives." Population Reference Bureau. Mar 1, 2001. https://www.prb.org/americanslivinglongernotnecessarilyhealthierlives/Accessed July 29, 2019.
 Makdisi, Tony and George Makdisi. "Extra-corporeal membrane oxygenation support: ethical dilemmas." Annals of Translational Medicine 5, no. 5 (2017): 112. https://doi.org/10.21037/atm.2017.01.38.
 Guttmacher A.P. "Progress and failure in population control." The Journal of Reproductive Medicine 8, no. 4 (1972): 159-161.
 Caldwell, John C. and Pat Caldwell. "High fertility in sub-Saharan Africa." Scientific American 262, no. 5 (1990): 118-125. https://doi.org/10.1038/scientificamerican0590-118.
 Barreca, Alan, Olivier Deschenes, and Melanie Guldi. "Maybe next month? Temperature shocks and dynamic adjustments in birth rates." Demography 55, no. 4 (2015): 1269-1293. https://doi.org/10.3386/w21681.
 Casey, George, Soheil Shayegh, Juan Moreno-Cruz, Martin Bunzl, Oded Galor, and Ken Caldeira. "The impact of climate change on fertility." Environmental Research Letters 14, no. 5 (2019). https://doi.org/10.1088/1748-9326/ab0843.
 Dreher, Rod. "Don’t get old in Belgium." The American Conservative. (Washington, D.C.) March 22, 2018. https://www.theamericanconservative.com/dreher/belgium-elderly-euthanasia/