Myth or fact

How long you live is mostly in your genes

Short answerNo

By GyroscopeUpdated November 14, 202610 sources

No. In 2,872 pairs of Danish twins, genes explained about a quarter of the differences in how long people lived, and a 2018 study of family trees linking about 400 million people put it under a tenth. A 2026 study that modeled out deaths from outside causes, such as accidents and infections, put it near half. Even that leaves half to everything else, and the habits are the part you control.

Why everyone believes it

Everyone knows a family where people live into their 90s, and a smoker who made it to 95. Long life does run in families. The question is how much of that comes from shared genes, and how much from shared kitchens, habits, money and neighborhoods.

A picture from daily life

Think of an accent. Kids sound like their parents, and you could guess a family from the way they talk. None of that is in their genes, since they just grew up in the same house. Lifespan runs in families partly the same way, through what families eat, how they move, whether they smoke and where they live.

What did the twin studies find?

Twins are the classic test. Identical twins share all their genes and fraternal twins about half, so if genes set lifespan, identical twins should die at closer ages. In 2,872 Danish twin pairs born from 1870 to 1900, genes explained about 26% of the differences in lifespan for men and 23% for women. A larger study of 20,502 Danish, Finnish and Swedish twins put it at 20 to 30%, and found that genes had little influence before 60 and more after.

Then in 2018 researchers used family trees that people had posted on Ancestry, 54 million of them. One connected tree alone joined about 406 million people. Their first estimates from blood relatives came out at 15 to 30%, like the twin studies. But people tend to marry people with similar lives, and that makes families look more alike than their genes alone would. Once the researchers allowed for this, the true heritability came out “well below 10%”.

What about the 2026 study that said half?

In January 2026, a team led by Uri Alon at the Weizmann Institute published a paper in Science arguing that lifespan is about 50% heritable. Their point was that many deaths in the old twin cohorts came from outside dangers that can strike at any age, such as infections and accidents, and those hide the genetic part. So they built a model that removed those “extrinsic” deaths and asked how heritable the rest, the “intrinsic” lifespan, would be. The answer came out at about 50 to 55%.

It’s a serious paper, and it doesn’t measure anyone’s genes. Outside experts pointed out that the result depends on the model chosen, since intrinsic deaths can’t be seen in the records, only estimated. And it’s about old twin cohorts born from 1870 to 1935.

Even taken at face value, about half isn’t “mostly”. And the authors’ own numbers show something useful. As fewer people died of infections and accidents across those birth years, the plain twin estimate roughly doubled. Genes explain more of the differences once the outside dangers are removed, which is what you’d expect.

Where genes do show

They do at the far end. In 444 families of people who reached 100, brothers were at least 17 times as likely to reach 100 themselves, and sisters at least 8 times. The researchers behind the New England Centenarian Study describe genes counting for more and more past 100, through many variants that each have a small effect. What do people who reach 100 have in common?

What do habits add?

Doesn’t 50% mean my genes decide half my years?