Counting the cost: Demographic shifts under China’s one-child policy

China’s One-Child Policy (OCP), as a measure to curb population growth, had profound and adverse demographic shifts, which we assess using a synthetic control method. We estimate the OCP prevented 50 million births, reducing the 0–14 age group by 3% and the young-age dependency ratio by 10.61%, while increasing the old-age ratio by 2%. It skewed gender balances, yielding 21 million excess males (52.5% of a 40 million surplus) and 10.69 million missing girls, with 5 extra boys per 100 girls in the 1991–2000 cohorts and 9 extra in 2001–2005. Robustness checks, including difference-in-differences and placebo tests, affirm these impacts, though post-1980 economic growth complicates attribution. These consequences, a leaner youth, an aging populace, and a male-heavy society, require policies to cater for single-child generations, for elderly care demands, and address gender inequity. This study offers critical insights for sustainable population strategies in China’s evolving landscape.