Abstract

Twin births are often construed as a natural experiment in the social and natural sciences on the premise that the occurrence of twins is quasi-random. We present population-level evidence that challenges this premise. Using individual data for 17 million births in 72 countries, we demonstrate that indicators of mother's health, health-related behaviors, and the prenatal environment are systematically positively associated with twin birth. The associations are sizable, evident in richer and poorer countries-evident even among women who do not use in vitro fertilization-and hold for numerous different measures of health. We discuss potential mechanisms, showing evidence that favors selective miscarriage.