This study explores how having more children affects parents’ longevity, with implications for family planning and social policy. Using 1940 U.S. Census data linked with Social Security death records and leveraging natural variation in child sex mix, the researchers find that each additional child is associated with about 5 months of reduced lifespan for women, while the effect on men is smaller and statistically insignificant (about 3.3 months). These results suggest that biological mechanisms may partly explain the link. Over the 20th century, fertility declines contributed approximately 11 months to women’s life expectancy gains—about 3.1% of the total increase in longevity during that period