The researchers say a person loses two months for every kilogram overweight they are— and seven years for smoking a packet of cigarettes a day.

    Unusually, the Edinburgh university team found their answers by analysing differences in people's genetic code or DNA. Finally they think it will show new ways of helping us to live longer.

    The group used the genetic code of more than 600, 000 people who are taking part in a natural experiment. If someone smokes, drinks, drops out of school and is overweight, it can be difficult to identify the impact of one specific unhealthy behavior.

    Instead, the researchers turned to the natural experiment. Some people carry mutations(变异) in their DNA that increase(食欲) or make them more likely to put on weight, so researchers were able to compare those programmed to eat more with those who were not. The research team also found specific mutations in human DNA that alter lifespan (寿命).

    Mutations in a gene (a set of instructions in DNA) that is involved in running the immune system could add seven months of life on average.

    People with a mutation that increased levels of bad cholesterol knocked eight months off lifespan.

    A rare mutation in a gene—APOE—linked to dementia reduced lifespan by 11 months.

    And one that made smoking more appealing cut lives by five months.

    Dr Joshi says that while genes do influence lifespan, “you've got even more influence” through the choices you make. Dr Joshi said, “We hope to discover genes affecting lifespan to give us new information about ageing and construct treatment tor ageing.”

    There are also some disease mutations that clearly affect lifespan and to destructive effect, such as the Huntington's gene. People with Huntington's often die in their 20s.

    However, in order to follow people until the end of their lives, many of the people studied were born before 1940.