Although long life tends to run in families, genetics has far less influence on life span than previously thought, according to a new analysis of an aggregated set of family trees of more than 400 million people.
The study suggests that the heritability of life span is well below past estimates, which failed to account for our tendency to select partners with similar traits to our own.
"We can potentially learn many things about the biology of ageing from human genetics, but if the heritability of life span is low, it tempers our expectations about what types of things we can learn and how easy it will be," said lead author Graham Ruby, from Calico Life Sciences -- a US-based research and development company.
"It helps contextualise the questions that scientists studying ageing can effectively ask," she added
Heritability measures how much life span can be explained by genetic differences, excluding differences like lifestyle, sociocultural factors and accidents.
While previous estimates of human life span heritability have ranged from around 15 to 30 %, in the new study it was likely no more than seven per cent, perhaps even lower.
For the study, published in the journal Genetics, the team used online genealogy resource with subscriber-generated public family trees representing six billion ancestors.
Removing redundant entries and those from people who were still living, they stitched the remaining pedigrees together included more than 400 million people, largely Americans of European descent.
Each of them was connected to another by either a parent-child or a spouse-spouse relationship.
They focused on relatives who were born across the 19th and early 20th centuries, and noted that the life span of spouses tended to be correlated, more similar than in siblings of opposite gender.
Comparing different types of in-laws, they found that siblings-in-law and first-cousins-in-law had correlated life spans, despite not being blood relatives and not generally sharing households.
The finding that a person's sibling's spouse's sibling or their spouse's sibling's spouse had a similar life span to their own made it clear that something else was at play, the researchers said.
The answer might lie in assortative mating. People tend to select partners with traits like their own -- in this case, how long they live, they explained.