How much of your lifespan is written in your genes?

Ask people why some folks live to 100 and most assume it's mostly luck of the draw — good genes. The data says otherwise. Classic twin studies estimate that only about 20–30% of the variation in human lifespan is heritable; the rest tracks with environment, behavior, and chance. In other words, roughly three-quarters of what separates a long life from an average one is not fixed at birth.

The genetic share may be even smaller than it looks. A 2018 analysis of over 400 million family-tree profiles found that couples tend to resemble each other in traits linked to longevity — a pattern called assortative mating — which had been inflating heritability estimates. Once researchers corrected for it, the true genetic contribution to lifespan looked closer to well under 10%.

There's an important nuance: genes matter far more at the extremes. Reaching exceptional old age (think 100+) does cluster strongly in families, because it takes a rare stack of protective variants to dodge disease that long. But for the typical person deciding how their 70s and 80s will go, modifiable factors — not smoking, staying active, sleep, diet, social ties — dominate the equation.