Three decades ago a famous study of Danish twins found that our genes “only moderately” influence how long we’re likely to live. But a new study more than doubles it, suggesting lifespan may be more genetically fixed than we thought. Early longevity studies analyzed groups of people who were born in a time of widespread extrinsic mortality. When they dialed extrinsic mortality down to zero, leaving only deaths caused by intrinsic aging processes, lifespan heritability roughly doubled. Considering extrinsic mortality has declined so much in the past century, however, Alon argues that “the higher number is more relevant” for people born today.