How long will I live? It’s more than just an existential question – think how useful the knowledge would be: we could decide whether to save, stick or blow our savings in one mad rampage. It’s only once we go into noticeable decline that can doctors help with predictions of how long we have left, and even then it’s a notoriously inexact science as they try to apply population data to an individual case.

So what do we actually know about longevity? Who will live and who will die at a given age? Is it all in the genes or do the life choices we make trump any genetic advantage? And how do we make sense of a new study that confirms that bright people live longer and that the difference in intelligence is genetic and not related to upbringing?

“We’ve known for at least 10 years that brighter people live longer,” says the study’s co-author Dr Rosalind Arden, research associate at LSE. “The current commonsense view is that brighter people tend to avoid crummy jobs and live relatively protected lives.” But recent twin research suggests that environmental differences don’t, in fact, account for the link between intelligence and longevity. Most twins, whether identical or not, share a common upbringing and childhood environment. So, if a big difference in IQ is found in non-identical twins that isn’t apparent in identical ones, it suggests that the difference is genetic – because all the twins have similar environments but only identical ones have the same genes. And Arden’s study found that, in non-identical twins, the brighter twin was more likely to live longer – whereas, in identical twins, there was no association between longevity and intelligence. Lifestyle choices, it seems, account for only 5% of the observed difference in intelligence – the other 95% being genetic.

“But we need to be humble with respect to our interpretation of these results,” warns Arden. “It’s scientifically interesting that intelligence is mostly genetic and it has implications for dementia and ageing. But you can’t extrapolate to individuals. Intelligence may be largely inherited, but how long you live depends on many factors and intelligence is only a small part. So if you have five kids who are all different, you can’t say that one will live to 90 but the other only until 40 because they perform differently in cognitive tests.”

Arden also points out that, because intelligence is largely genetic, any early advantage conferred by reading to your kids and putting them through cello lessons won’t affect their long-term cognitive tests. “By the age of 16, or 14 in our research, the influence of upbringing is zero. The child will remain enriched, which is arguably more important than test scores, but any early advantage in terms of intelligence ranking disappears.

So the question remains: why should being more intelligent make you live longer? “It could be that people whose genes make them brighter also have genes for a healthy body. They may have organs that are well put together – brains, liver, kidneys. Or intelligence and lifespan may both be sensitive to overall mutations, with people with fewer genetic mutations being more intelligent and living longer,” says Arden. “The next step needed to probe the causes are good animal studies. If several species repeat the pattern of brighter individuals living longer, without confounding by lifestyle factors, it would be fascinating.”

So what of the other determinants? Again, genetics come into play. Twin studies suggest that 20-30% of the variation in lifespan is genetic and the rest is environmental. Many different genes are likely to contribute. And while it’s helpful to come from a family of long-lived individuals, lifestyle choices and environmental factors trump genetics. Smoking, unsurprisingly, is the single biggest lifestyle factor that can affect how long you live. A UK study in 1996 showed that men who never smoke have a 78% chance of reaching 73. Those who start smoking by the age of 20 and never stop only have a 42% chance. According to a 2014 Swiss study, alcohol intake, diet and exercise all play a significant but smaller part in maximising longevity.

The main sources of information for longevity and the factors associated with it are cohort studies that follow groups of people over many years. However, though researchers do their best to correct for confounding factors, it can be hard to prove or distinguish between cause and result. For instance, walking every day is associated with living longer in these studies – but then clearly only fitter people can get out every day in the first place.

Other cohort studies have examined the link between income and longevity. Professor Jochanan Stessman of Hadassah Hebrew University Medical Center is the founder of a study that has followed up residents of West Jerusalem who were 70 years old in 1990. The cohort is particularly interesting because they are a diverse group from a wide range of cultural and socio-economic backgrounds, many of whom have experienced the Holocaust, emigration and wars. The study found that individuals who are well off and well educated live longer; ethnicity and country of origin seem to make little difference.

Another important factor appears to be social rather than physical. “Until middle age, traditional risk factors like smoking, high blood pressure, high cholesterol and diabetes are important. But, as you get older, they become less important. What makes the difference is staying physically, mentally and socially active,” says Dr Jeremy Jacobs, geriatrician at the Hadassah Medical Center. Feeling useful, reading and getting out of the house every day are all features of people who will live longer. “Older people should use their hearing aid and get their cataracts sorted out because being able to hear and see as well as possible are important factors in avoiding isolation.”

A final, obvious point is that we are much better at avoiding dying young. Our genetic predisposition hasn’t changed, but we have conquered many of the environmental hazards that used to carry us off before we lived out our genetically allocated time. Childhood immunisations, clean water, enough food, less overcrowding, and, to a lesser extent, modern medicine have all played their part. David Sinclair, director at the International Longevity Centre UK, says: “We’ve massively reduced the likelihood of dying under 65. Now we’re reducing mortality in over-65s. We’re less likely to die of a heart attack and stroke – but now more of us will get cancer and dementia as we live longer.”