Fertility decline has no single cause and no single consequence. Some causes are nevertheless quite obvious. Contraception prevents conception. Abortion converts pregnancies that would otherwise often end in births into non-births. Education, paid employment, and later marriage consume years that were once available for childbearing and raise the opportunity cost of motherhood. Housing, culture, health, and the expectations attached to parenting shape the result further. The mix differs from one country to another, but complexity is not a reason to talk around the mechanisms.
The chapters move from history and measurement into family formation, economics, biology, culture, public policy, and the long future. They examine what happens inside a household and what happens to a pension system; why two people may postpone a first child and why a country may struggle to staff hospitals thirty years later. These are different levels of analysis. They belong together because demographic change is the accumulated result of intimate decisions made within economic, social, and biological limits.
Different paths to the same statistic
A fertility rate cannot tell us whether people are living the lives they wanted. Two societies can reach the same number through very different combinations of early parenthood, delayed births, small families, infertility, childlessness, unstable partnerships, or migration. Two people counted within the same national average may have almost nothing else in common.
This book therefore distinguishes among voluntary childlessness, postponed childbearing, unintended childlessness, infertility, and the gap between desired and achieved family size. Those distinctions are morally and practically significant. Someone who does not want a child does not have the same problem as someone who cannot find a partner, cannot afford a suitable home, or discovers too late that reproductive medicine cannot fully compensate for age.
The 2025 UNFPA report on reproductive agency is useful here because it asks about the family people wanted rather than treating every birth or non-birth as interchangeable. Its 14-country survey found both unintended pregnancy and unrealized desires for children. I will use evidence like this carefully. Stated intentions can change, surveys depend on wording and timing, and no sample can settle what billions of people want. Still, a society should notice when freedom on paper and attainable choices in practice begin to separate.
Three questions
The first question is why fertility has fallen. The answer requires history because today’s choices were made possible by enormous gains in survival, education, income, and reproductive control. It requires economics because children need time, space, and money. It requires attention to culture because people form families according to ideas about adulthood, marriage, work, sacrifice, and the future. It also requires biology because delayed childbearing changes what is possible even when intentions remain the same.
Causes we can count
The scale of reproductive control is enormous. In 2022–2023, the National Survey of Family Growth found that 54.3 percent of American females ages 15–49 were currently using contraception. The largest categories were female sterilization at 11.5 percent, the pill at 11.4 percent, long-acting reversible methods at 10.5 percent, and male condoms at 7.1 percent. Globally, the UN estimates that the number of women using modern contraceptive methods rose from 552 million in 1994 to 871 million in 2024. This access allows people to avoid births. That is its purpose, and it helped make low fertility possible.
Abortion has a similarly direct relationship to births, even though legal regimes, intentions, and counterfactual behavior complicate any estimate of its net effect on a national fertility rate. The CDC received reports of 613,383 legal induced abortions in 2022, excluding several states that did not report. Guttmacher’s more comprehensive provider census estimated 1,037,000 abortions in the formal U.S. health-care system in 2023. The World Health Organization estimates about 73 million induced abortions worldwide each year. These are not marginal numbers in a book about missing births.
Paid work is less mechanically simple, because women may work less after having children and may have fewer children because they work. The association is still large. A Bureau of Labor Statistics study of women born from 1957 through 1964 found 2.5 children by age 46 among women who had worked full time in no more than a quarter of the years from age 25 to 46, compared with 1.4 among women who had worked full time in more than three quarters of those years. Later chapters will separate selection, timing, education, and causation. I won’t pretend the conflict between the hours required by a career and the hours required by several children is mysterious.
The second question is whether fertility can rise again. Governments have tried cash payments, tax relief, childcare, parental leave, housing support, marriage incentives, and public campaigns. Some policies make family life easier, and that is worthwhile even when the effect on birth rates is modest. The harder question is whether policy can reverse a durable social pattern without treating people as instruments of the state.
The third question is how well societies can adapt if fertility remains low. The OECD’s work on aging populations describes a broad menu: higher employment, longer working lives, pension reform, migration, family policy, and productivity growth. Technology may allow fewer workers to produce more. Healthier old age may make the familiar boundary between worker and retiree less useful. Immigration can give a country time and talent. It also shuffles people from one country to another, and migrant fertility generally converges toward the host population, often substantially by the second generation. It cannot be a permanent answer to a global decline.
These questions are often collapsed into one argument. A person who doubts that governments can raise fertility may still believe societies should remove needless barriers to family formation. A person who thinks adaptation can work for decades may still worry about repeated global contraction over centuries. A policy can improve family well-being without moving the national fertility rate very much. I will try to keep those claims separate.
What the book is trying to protect
I am not looking for a single ideal population size, and I do not think prosperity can be measured by how many babies are born. A country with more births can still be poor, unjust, violent, or hostile to family life. A smaller society can be peaceful, productive, beautiful, and generous. The number alone does not tell us whether people are flourishing.
The concern is continuity rather than maximization. Families pass on life, but also language, memory, habits, faith, craft, property, obligations, and affection. Schools, firms, laboratories, governments, and neighborhoods do similar work at a different scale. Each generation receives institutions it did not create and decides, usually imperfectly, what to repair, what to discard, and what to hand on.
That inheritance gives the book its longest horizon. The UN’s current medium projection stops in 2100, as it should. Beyond that point, demographic arithmetic becomes increasingly sensitive to assumptions no model can defend with much confidence. Civilizations still have to think beyond a forecast’s endpoint. Infrastructure, constitutional orders, scientific traditions, ecological restoration, and settlements beyond Earth are all multigenerational projects.
So the scope of this book is wide, but its standard is fairly simple. I want to ask which arrangements help free people form the families they hope for, which adaptations allow older and smaller societies to remain humane and prosperous, and where adaptation may cease to be enough. The point is not to subordinate a person’s life to the future. It is to remember that the future will be lived by persons too.
Citations
- State of World Population 2025: The Real Fertility Crisis, United Nations Population Fund, 2025.
- Ageing Populations: Their Fiscal Implications and Policy Responses, Organisation for Economic Co-operation and Development, 2025.
- World Population Prospects 2024, United Nations Department of Economic and Social Affairs, Population Division, 2024.
- “Current Contraceptive Status Among Females Ages 15–49: United States, 2022–2023”, National Center for Health Statistics, 2025.
- “The New Landscape of Fertility and Family Planning 30 Years After Cairo and Beijing”, United Nations Department of Economic and Social Affairs, 2025.
- “Abortion Surveillance—United States, 2022”, Centers for Disease Control and Prevention, 2024.
- “Despite Bans, Number of Abortions in the United States Increased in 2023”, Guttmacher Institute, 2024.
- “Abortion”, World Health Organization, 2025.
- “Fertility Patterns of Native- and Foreign-Born Women”, Monthly Labor Review, U.S. Bureau of Labor Statistics, 2016.
- “Migrant family building: Recent evidence and implications”, in International Migration Outlook 2023, Organisation for Economic Co-operation and Development, 2023.