The average life expectancy in many developed countries is around 80 years old. Some individuals have famously lived for much longer - the oldest person who ever lived was 122 years old!

Whilst death is inevitable, longevity varies drastically between individuals. What is the secret to a long life? Is our longevity already encoded in our DNA or is it determined by our lifestyle?

We asked 11 experts in ageing, cell biology and genetics: "Is longevity primarily determined by genetics?". Here's what we found out.

Longevity in comparison to what?

Experts had two different interpretations of the question:

1) 'Is the longevity of humans as compared to other species primarily determined genetics?'

2) 'Is the longevity of some individual humans as compared to other humans primarily determined by genetics?'

Genetics determines cross-species lifespans

Different animal species have very different lifespans. The Greenland shark is able to live to be 400 years old, whilst some species of mayfly only live for 5 minutes. The reason for these differences is genetics.

Professor David Gems, an expert in ageing from University College London, says "the question could mean: are the upper limits of longevity in humans as a species primarily determined by genetics, in which case the answer is 'near certain'. For example, the maximum lifespan of human beings is approximately twice that of our closest relatives among the higher primates, such as chimpanzees and gorillas."

Lifestyle more important than genes across humans

Dr Gems says "If one takes [the question] to mean: are the differences in lifespan between individual people primarily determined by genetics, then the answer is 'extremely unlikely'." Most of the experts agreed with Dr Gems. Whilst genetics plays a role in longevity, it is not the primary determining factor.

Professor Dame Janet Thornton, an expert in anti-ageing and cell biology and previous director of the European Bioinformatics Institute says that "genetics accounts for less than 30 percent of the effect - but it is true that longevity tends to run in families - i.e. some families have many very old people".

It can be difficult to determine if the existence of families with many very old people is due to genetics or environment, as often family members adopt similar diets and lifestyles. Studying the DNA of these long-lived people could tell us more.

Professor Ken Parkinson, an expert in anti-aging and oncology from Queen Mary University London says "many groups are trying to understand this by sequencing the DNA of centenarians and supercentenarians and performing genome-wide analysis."

The lifestyle effect on longevity is clearly apparent when we look at how average life spans have increased over hundreds of years due to increased accessibility of clean water, food and medical care.

Counter-intuitively, it has been shown that restricting calorie intake could be linked to longevity in humans. Another lifestyle factor is exercise. Even light exercise for 15 minutes a day has been shown to increase life expectancy by around 3 years.

Individual longevity between humans is affected by genetics, but not primarily. Professor Lorna Harries, an expert in genetics and cell biology from Exeter University summarizes that "genetics can set you up for a long life, but you then need to do the right things to realize that."

The takeaway

Differences in longevity between humans is not primarily determined by genetics. Regular exercise and a healthy diet may be the 'secret' to longevity.

Article based on 11 expert answers to this question: Is longevity primarily determined by genetics?

This expert response was published in partnership with independent fact-checking platform Metafact.io. Subscribe to their weekly newsletter here.