How much do genes decide how long we live? For many years, scientists believed that genetics explained only about 20 to 25 per cent of differences in human lifespan. The rest was thought to depend on lifestyle, environment, and chance. A new study published in Science now suggests that genes may play a much bigger role in modern times.
Researchers say the change is linked to how causes of death have shifted over the last century. In the past, many people died young due to infections, accidents, poor sanitation, or lack of medical care. These are called external or “extrinsic” causes. Today, especially in developed countries, most people die from ageing-related illnesses such as heart disease, dementia, and other chronic conditions. These are known as internal or “intrinsic” causes.
To better understand this shift, scientists studied large groups of Scandinavian twins. They carefully removed deaths caused by accidents and infections from their analysis. They also looked at twins raised apart and siblings of very long-living people in the United States. When deaths from external causes were excluded, the genetic influence on lifespan increased sharply to about 50–55 per cent.
This does not mean genes have suddenly become stronger. Instead, it reflects how living conditions have improved. As factors like vaccination, nutrition, clean water, and healthcare have reduced environmental risks, genetic differences have become more visible.
The study explains this with a simple example. In the past, height was strongly affected by poor nutrition and childhood illness. Today, because most people get enough food, height differences are mostly due to genes. Nutrition still matters, but fewer people are limited by it.
The same idea applies to lifespan. When environmental differences shrink, genetic influence appears larger. Scientists call this “heritability,” and it changes depending on time, place, and conditions.
Importantly, the study does not say genes decide half of your fate. Lifestyle, healthcare, and environment still matter greatly. People can overcome weaker genetics with healthy habits, while others benefit from strong genetic protection.
The researchers say understanding how genes and environment work together may help explain why some people live much longer than others. In the end, both matter—and they matter together.
