353: Masculinization Reverses Sex Differences in Fertility

Schubert HA et al., PNAS - This episode reviews a global analysis showing that shifts in population sex composition have reversed historical sex gaps in fertility. Using UN World Population Prospects 2024 data and regression and standardization methods, the authors estimate male total fertility rates and project widening differences through 2100 driven by masculinized reproductive-age populations. Key terms: male fertility, sex ratio, total fertility rate, sex-selective abortion, population structure.

Study Highlights:
The authors estimate male and female total fertility rates using UN WPP2024 data combined with a regression-based model and demographic standardization. They identify a global crossover in 2024 when female TFRs first exceeded male TFRs and project growing disparities through 2100, especially in East Asia. Key drivers are higher sex ratios at birth, declining mortality, and a narrowing male–female mortality gap, with sex-selective abortion amplifying effects in some countries. The analysis highlights social consequences such as rising male childlessness and offers policy recommendations to mitigate sex imbalances.

Conclusion:
Masculinization of reproductive-age populations has flipped historical fertility patterns so that female TFRs now often exceed male TFRs, a gap expected to widen in many regions and to carry social and policy implications related to childlessness and partnership markets.

Music:
Enjoy the music based on this article at the end of the episode.

Article title:
Masculinization of populations reverses sex differences in fertility

First author:
Schubert HA

Journal:
PNAS

DOI:
10.1073/pnas.2533317123

Reference:
Schubert HA, Spoorenberg T, Dudel C, Skirbekk VF. Masculinization of populations reverses sex differences in fertility. Proc Natl Acad Sci U S A. 2026;123:e2533317123. doi:10.1073/pnas.2533317123

License:
This episode is based on an open-access article published under the Creative Commons Attribution 4.0 International License (CC BY 4.0) – https://creativecommons.org/licenses/by/4.0/

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Episode link: https://basebybase.com/episodes/353-masculinization-populations-fertility

QC:
This episode was checked against the original article PDF and publication metadata for the episode release published on 2026-04-30.

QC Scope:
- article metadata and core scientific claims from the narration
- excludes analogies, intro/outro, and music
- transcript coverage: Audited the transcript sections describing historical sex differences in fertility, the 2024 crossover, drivers (birth sex ratio, mortality decline, sex-selective abortion), the age-gap Model 3 estimation approach, regional variations (East Asia vs Sub-Saharan Africa), war shocks, and policy implications.
- transcript topics: Historical fertility gender gaps and denominator effects; Birth sex ratio and sex-selective abortion; Mortality decline and sex mortality gap; Model 3 age-gap estimation of male TFR; Global crossover year 2024 and 2100 projections; Regional variations: East Asia vs Sub-Saharan Africa

QC Summary:
- factual score: 10/10
- metadata score: 10/10
- supported core claims: 7
- claims flagged for review: 0
- metadata checks passed: 4
- metadata issues found: 0

Metadata Audited:
- article_doi
- article_title
- article_journal
- license

Factual Items Audited:
- Global crossover year: 2024
- 2100 projection: <10% of countries have higher TFRm
- Three drivers of masculinization: birth sex ratio, mortality decline...

Chapters

• (00:00:12) - The feminization of the population
• (00:05:45) - The demographic structure of human populations
• (00:07:15) - The Real Story Behind Global Fertility
• (00:12:47) - Maternal mortality in the US and Europe
• (00:13:19) - How Does War Affect Male Fertility?
• (00:17:27) - What Actually Happens to Men When They're Out of the Marriage
• (00:21:46) - The Future of Families Is Shaping
• (00:23:42) - A Place to Be After the Crossover