Your genes matter more for lifespan now than they did a century ago-here’s why

How much do your genes determine how long you'll live?

- KARIN MODIG

It’s a question that fascinates us, and one that’s been debated for decades. For years, the answer seemed settled — genes account for about 20-25 per cent of the variation in human lifespan, with the rest down to lifestyle and environment.But a new study published in Science has challenged this view, suggesting the genetic contribution might be considerably higher.

The reason, according to the researchers, is that previous estimates failed to account for how the causes of death have changed over time. A century ago, many people died from what scientists call extrinsic causes ~ accidents, infections and other external threats.

Today, in developed countries at least, most deaths result from intrinsic causes: the gradual wearing out of our bodies through ageing and age-related diseases like dementia and heart disease.

To get a clearer picture, the research team analysed large groups of Scandinavian twins, carefully excluding deaths from external causes. They also studied twins who were raised apart and siblings of centenarians in the US.

When they stripped away deaths from accidents and infections, the estimated genetic contribution jumped dramatically — from the familiar 20-25 per cent to around 50-55 per cent.

The pattern makes sense when you look at individual diseases. Genetics explain much of the variation in dementia risk, has an intermediate effect on heart disease, and plays a relatively modest role in cancer. As environments become more favourable, populations age and diseases caused by the ageing process itself become more common, the genetic component naturally appears larger.