What new twins study reveals about genes, environment and longevity

Why do some people live to 100 while their sibling dies decades earlier? Is it luck, lifestyle, or something written into their DNA?

Relative to many other species, humans are particularly long lived, but there is an ongoing argument about how much of our long lifespan is shaped by our genes and how much to our environment. It’s the old “nature versus nurture” debate.

Researchers have repeatedly used large population studies to estimate how much genetics influences longevity. Historically, these studies have found relatively modest effects, typically around 25% to 33%, with some estimates as low as 6-16%.

A recent study published in Science challenged this trend, revising the estimate upward to about 50% by accounting for changes in external causes of death – such as accidents and infectious diseases – and separating the effects of genetics and environment in large historical cohorts of twins.

We know that individual genes affect lifespans in different species. A single mutation in the gene coding for the insulin sensor of worms would cause them to double their lifespan. Since that 1993 discovery, scientists have experimentally extended the lifespans of flies and mice, and even found hints of similar effects in long-lived humans.

However, the effect of this single gene variation seems to be lessened as the species becomes more long lived, so don’t expect a single gene mutation to suddenly cause 200-year human life expectancies. Also, these were gene mutations affecting the sensitivity of insulin and insulin-like growth factor hormones – in other words, the mutations seem to mimic the metabolic effects of a healthy diet and regular exercise.

Perhaps an obvious statement to make, but many of our body’s traits, including longevity, are the sum of all our inherited genes, not just a single gene. But how much is genetics and how much is lifestyle is the open question.

This amount is more than an interesting number. If genetics mostly controls how long we live, then new anti-ageing treatments and lifestyle changes won’t help much. But if genetics plays a smaller role, then what we do and the treatments we use could make a bigger difference in how long we live.

Nature’s perfect experiment

To tackle this question, the authors of the Science paper used data from the Swedish Adoption/Twin Study of Ageing. Because it includes a rare set of twins raised apart, the data makes it easier to tease apart the effects of genes and environment, creating something close to a natural experiment in humans.

By studying monozygotic (“identical”) twins, or people with identical genomes born between 1900 and 1935 and attempting to correct for rapidly changing external influences of health and mortality during this period, the authors conclude that the inherited influence of lifespan is about 50%.

Put another way, about 50% of your potential lifespan is given to you by your parents, and the other 50% is the environment you live in. Things such as exercise, nutrition, sleep, stress, pollution and infectious disease exposure all fall into this external category.

The researchers then validated their models using data from populations in Denmark and the US. However, this also means the study populations were largely white, wealthy and European. Including more diverse populations will be important for determining how well these findings apply to humanity as a whole.

The reason that the authors put forward for their number being so much higher than others is their accounting for the effects of changing external influences on longevity, things such as improving sanitation and medication.

Indeed, if they don’t correct for external causes of death, their model finds numbers in the 20-30% range, or much closer to prior estimates. As the authors note, many health traits seem to be about 50% heritable, so there’s an internal logic of longevity as a trait following this trend.

These estimates could still change. Longevity studies in humans are time consuming, relying on historical records or following populations over roughly 100 years as people live their normal lives. As the authors note: “Heritability is a statistic that applies to a particular population in a particular environment at a particular time.”

This doesn’t mean that your personal actions aren’t helping you – this debate probably isn’t over yet. This is the largest estimate of the effect of genetics on longevity to emerge recently in the scientific literature, but even if genes account for about half our lifespan’s story, the other half is still being written every day.

Bradley Elliott receives funding from the Physiological Society, the British Society for Research on Ageing, the Altitude Centre, and private philanthropic individuals, and has consulted for industry and government on longevity research. He is on the Board of Trustees of the British Society for Research on Ageing.