This study explores how birth order affects longevity, finding that later-born siblings live 1–3 months less than their earlier-born counterparts. The effect is strongest when comparing siblings within the same family, suggesting that parental favoritism or diluted resources may play a key role. The impact is more pronounced among white individuals but does not vary by gender, family income, or maternal education, pointing to widespread within-family inequalities. The longevity gap, while small in absolute terms, accounts for 1.5–4.3% of the 20th-century gains in life expectancy, highlighting how early-life dynamics can shape long-term health