Dr. Shahar Madjar, Journal columnist

How long are you going to live? Is your longevity determined mostly by genetics?

One patient of mine is 93 years old and still maintains an agile posture and a sharp sense of humor. His older brother was also a patient of mine. He died at 98. Both told me that extreme longevity was the rule in their family, with only a few exceptions. Their case seems to suggest that genetics plays a major role in life expectancy.

But is this true for most of us? And if our good fortune is largely determined by our genes, what is the point of maintaining a healthy weight, exercising, and insisting on healthy habits?

Ben Shenhar from the Department of Molecular Cell Biology at the Weizmann Institute of Science, and his colleagues, tried to answer these questions in a study published in the prestigious journal Science.

For decades, scientists believed that genetics played only a modest role in determining how long humans live. Most estimates suggested that heredity explained about 20-25% of the variation in lifespan. The rest was attributed to lifestyle, environment, chance, and history. Don’t smoke. Exercise. Eat well. Make friends and keep good company. Avoid accidents and wars. Stay away from crowded places during pandemics. In short, it helps to live right in the right place at the right time.

Those estimates were based largely on population-level data, family studies, and twin registries. Researchers compared how long parents lived compared to their children, or how lifespan clustered within families.

But there was a fundamental problem hiding in plain sight.

A person can die young because of a car accident, war, infection, or random violence. Another person might live into their nineties and die slowly from organ failure related to aging. From a biological perspective, these are entirely different events.

Older studies treated all deaths the same, regardless of cause. As a result, a genetically robust individual who dies at 30 in a car crash looks–statistically–like someone with a short biological lifespan. Genetics was being judged on outcomes it had no chance to influence.

In other words, lifespan itself–the age at death–was assumed to reflect the biology of aging. But many deaths do not.

The new study, published on January 29, 2026, challenges that assumption. It does not identify new longevity genes. Instead, it asks a more basic question: what are we actually measuring when we measure lifespan?

Shenhar and his colleagues asked what would happen if deaths caused by intrinsic aging were separated from deaths caused by external events.

Rather than treating lifespan as a single number, they modeled mortality as the sum of two forces. Intrinsic mortality reflects death driven by internal biological aging processes. Extrinsic mortality reflects death caused by accidents, infections, violence, and other external hazards.

The researchers reanalyzed large, well-established twin datasets using updated mortality models. Instead of simply correlating age at death, they used statistical techniques that account for how people died, not just when.

Why twins? Because twin studies act like a natural experiment that cannot be ethically or practically reproduced. Identical (monozygotic) twins share virtually 100% of their genes, while fraternal (dizygotic) twins share about 50%, similar to ordinary siblings. Crucially, both types usually grow up at the same time, in the same household, exposed to similar environments.

If identical twins resemble each other in lifespan more than fraternal twins do, that difference can be attributed–statistically–to genetics. This is why twin studies are so powerful for separating genetic influences from environmental ones.

When external causes of death were mathematically accounted for, something striking emerged. The genetic contribution to intrinsic lifespan rose dramatically–from about 20-25% to roughly 50-55%.

This does not mean genes determine your fate. It means that when aging is allowed to reveal itself without interference from randomness and history, biology matters far more than we previously realized.

Does this mean you should smoke, avoid exercise, eat junk food, engage in risky behaviors, and insert yourself into every conflict possible? Absolutely not.

Genetics may play a larger role in longevity than we once thought, but environmental and behavioral factors still exert enormous influence–especially on whether we survive long enough for our genes to matter at all.

EDITOR’S NOTE: Shahar Madjar, MD, MBA, is a urologist and an author. He practices at Schoolcraft Memorial Hospital in Manistique, and in Baraga County Memorial Hospital in L’Anse. Find his books on Amazon or contact him at smadjar@yahoo.com.