Mary Belle Roach and Mae Belle Powell (known as the ‘Wallace sisters’ because of their maiden names) are identical twins. Throughout their lives they’ve rarely been apart. They live together in the Kentucky house in which they grew up. They dress alike and do most things together. They’ve both outlived their husbands. And in May 2016 they became centenarians together.

Why do they think they’ve both reached 100?

“Good genes. Our mother lived to be 97,” says Mae Belle. But she adds: “We didn’t smoke cigarettes. We didn’t smoke that pot. We never drank beer.”

Let’s take that first answer (we’ll get to the tobacco, drugs and alcohol later). Is the Wallace sisters’ longevity down to their genes?

Plausibility rating: 10 out of 10.  Genes play a big role in our longevity and it appears to become even more important the older we get. Let’s try and tease out the facts.

The first place to turn are studies of identical twins like the Wallaces. Twins share identical genetic make-ups so if genes were to determine everything about our longevity we’d expect both twins to have the same length of life.

But they don’t: some die much earlier than their twin, some much later. In fact, this major study of twins estimated that just 20%-30% of longevity is inherited. The rest, it said, is down to their environment.

However those studies weren’t looking just at centenarian twins but at twins who died at all ages. Could it be that those who make it to 100 share a different genetic make-up to those that don’t, perhaps with a higher amount of heritability?

Almost certainly yes. One strong piece of evidence for this is a major study of over 2,000 family members of centenarians. It found that, compared to the population as a whole, male children of 100-year-olds were at least 17 times as likely to get to 100 themselves, while females were at least eight times as likely.

This suggests that genes are playing a much larger role than the 20-30% in the population as a whole. It helps to explain why extreme longevity often clusters in families (the Guinness Book of Records award for the highest combined total for nine siblings is an incredible 818 years, awarded to the Melis family of Sardinia in 2012). And certainly centenarian twins turn up much more frequently than you might expect if genetics played a limited role.

Why might that be? Gerontologists talk about something called the ‘compression of morbidity’ hypothesis. Basically, this is the tendency for some people to stave off illness and disability until much later in their lives than most people.

Some achieve it because they look after themselves. This study of Seventh Day Adventists in the United States suggests that diet, exercise and other factors can help get the ‘average’ person well into their 80s, adding up to 10 years to longevity.

However not only do centenarians get this benefit for longer, many of them seem to have it granted them naturally: it’s in their genes. This research in Japan for example found that at 92% of people who went onto become centenarians were still living independently at 92. That genetic tendency may even include some degree of protection from habits that are usually pretty disastrous for health, like smoking. This research for example claims to have identified a gene that makes smokers less likely to die from their habit than the rest of the population (though it’s by no means definitive: we cover the subject in a bit more depth here). And we’ve previously seen that very long-lived people like centenarians are less likely to die of conditions like cancer.

So it’s fairly clear that genetics does play a large role in whether you’ll live to 100. However there plenty of reasons why we should be wary of assuming that genes completely determine our lifespan. For a start, despite intense efforts, scientists have struggled to find a gene or even a set of genes that correlate closely with longevity. And even if we can identify the genes that aid longevity, we may find that they are shared by quite a large number of people.

Secondly, our environment influences how our genes express themselves, including turning individual genes on and off. So despite having identical genomes, even identical twins like the Wallace Sisters may differ across a range of factors such as susceptibility to illness.

And finally, lots of other factors can also influence how long we live. As we’ve seen in the previous 48 ways of living to 100, these include what we eat and drink, how much exercise we take and the strength of our social networks. Changing these factors alone can make a significant difference to our longevity. Which reminds us that the Wallace sisters wisely ascribed their longevity not just to their genes but also to not drinking and not smoking (especially ‘that pot’).

Overall, our genetic make-up is perhaps best seen as strongly shaping our ‘potential’ longevity, with our environment and own behaviours then influencing how much of that potential we actually achieve. But the honest truth is that we really don’t know for sure.

photo credit: Chromosomes and DNA double helix via photopin (license)