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.

We identify dynamic impacts on domestic violence (DV) of the staggered imposition and lifting of lockdown across Chile’s 346 municipalities. Lockdown increases DV helpline calls and shelter occupancy without increasing DV police reports. These results are consistent with lockdown raising incidence and creating barriers to reporting. Once lockdown is lifted, shelter occupancy falls and police reports surge, but helpline calls remain elevated in line with state dependence in DV. We identify male job loss as a mechanism driving DV. Our findings accentuate controversy around welfare impacts of lockdown mandates. Adverse impacts of lockdown on DV are mitigated by cash transfers.