How much do your genes determine how long you will live? It is a question that has fascinated scientists 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 shaped by lifestyle and environment.
However, a new study published in Science challenges this long-held view, suggesting that the genetic contribution to lifespan may be considerably higher.
The researchers argue that earlier estimates failed to account for how 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, at least in developed countries, most deaths result from intrinsic causes: the gradual wearing out of the body through ageing and age-related diseases such as dementia and heart disease. To get a clearer picture, the research team analysed large groups of Scandinavian twins, carefully excluding deaths caused by accidents and infections. They also examined twins raised apart and siblings of centenarians in the United States.
When deaths from external causes were removed, the estimated genetic contribution rose sharply, from the familiar 20-25 per cent to around 50-55 per cent. This pattern aligns with what is known about individual diseases. Genetics explain much of the variation in dementia risk, have an intermediate effect on heart disease, and play a relatively modest role in cancer. As environments improve, populations age, and diseases linked to ageing become more common, the genetic component naturally appears larger. Crucially, a higher estimate does not mean genes have suddenly become more influential, nor does it mean individuals can only control half their chances of reaching old age. What has changed is the environment, not our DNA.
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