Two-thirds of the world's population currently lives in countries where the birth rate is below two children per two adults (replacement level), which is the threshold for depopulation.

South Korea currently has a fertility rate of 0.78 children per woman, the lowest globally, which means that within two generations South Korea will have 1/7th the number of babies it is currently having.

Sub-Saharan Africa currently has an average birth rate over 4, down from 5 a couple decades ago and 6 a couple decades before that, with fertility expected to continue declining as female education increases (consistent with pattern in India and Kenya).

The conversation about population decline and stabilization should begin now, decades before the projected population peak in 2084, to allow societies time to gather data, develop understanding, and build social movements, similar to how climate change discourse took 60 years to develop from initial warnings to current policy focus.

Paul Ehrlich's 1968 book 'The Population Bomb' was published in the year global population growth rate peaked, making it unfortunate timing, as the very thing he was warning against was already beginning to reverse.

Certain large-scale existential risks (asteroid impacts, pandemics requiring vaccine development) have high fixed costs and are more likely to be managed effectively by larger populations with more scientific capacity than smaller populations.

The year when the most babies were born globally was 2012, when 146 million babies were born worldwide, and the number of births per year has been declining ever since.

Financial incentives and subsidies (such as child tax credits, free university, or cash payments for childbirth) have not been shown to significantly increase lifetime fertility rates, only to shift the timing of when people have children, not whether they have them.

In India, women voluntarily choose to stop having children through permanent contraceptive surgery (tubal ligation) in their 20s after one to three children, demonstrating that low birth rates reflect deliberate choice, not inability to conceive.

Latin America has a birth rate of 1.8 (below replacement) and is predominantly Christian (90%), yet has low fertility, demonstrating that religion is not the primary driver of fertility decline.

Population stabilization at any level (whether 10 billion, 8 billion, 6 billion, or 1 billion) will eventually require an average birth rate of two; therefore, the goal of stabilization requires solving the problem of bringing fertility up to replacement level.

India currently has a birth rate below two children per two adults, and young women even in the poor state of Uttar Pradesh (where Spears conducts research) say they want an average of 1.9 children.

Low and falling birth rates appear to be a convergent phenomenon—many different headwinds work against having large numbers of babies—rather than driven by a single cause like capitalism, religion decline, marriage decline, or female labor force participation, as exceptions exist for each single-cause theory.

The opportunity cost of having children—what one gives up to have a child—has increased dramatically as work quality has improved, entertainment options have expanded (especially through technology), and the pleasantness of not having children has increased, while the discomforts of parenting have not substantially changed.

Of 26 countries where completed cohort fertility (lifetime average births per woman) has ever fallen below 1.9 since the mid-twentieth century, zero of them have ever seen fertility rates climb back up to two.

Environmental problems have been solved historically through technological and policy changes (elimination of CFCs for ozone, unleaded gasoline for lead pollution, sulfur dioxide regulation for acid rain, coal regulation in China for particulate pollution), not through population reduction.

China's particulate air pollution (PM 2.5) fell by approximately 50% in the decade following 2013 (the 'air apocalypse' year) despite the Chinese population growing by 50 million people, demonstrating that population growth and environmental improvement can coincide.

Food availability per capita has increased on every continent and in every decade, and nutrition has improved globally (e.g., average children in India are taller than they were decades ago and have less disease), contradicting the Malthusian prediction of hard limits on population-supporting capacity.

A larger population with a larger economy can generate greater negative emissions capacity (planting trees, rock weathering for carbon sequestration, etc.) than a smaller population, so under some climate models, larger population in the long run leads to lower temperatures once the world reaches net-zero carbon emissions.

Under endogenous growth theory, improvements in living standards come from knowledge created by people that can be learned and built upon by others; knowledge is non-rivalrous (using an antibiotic formula does not use it up for others), so larger populations may generate more innovation and more benefit from cumulative knowledge.