Although longevity tends to run in families, genetics has a far less influence on life span than previously thought, according to a new analysis of a collection of more than 400 million family trees.
This study shows that heritability of life span is far below past estimates, which failed to take into account our tendency to choose partners with features similar to our own.
"We can potentially learn a lot about the biology of aging from human genetics, but if the heritability of the life span is low, it calms our hopes about the types of things we can learn and how easy it is," said lead author Graham. Ruby, from Calico Life Sciences – a US-based research and development company.
"This helps contextualize questions that scientists studying aging can effectively ask," he added
Heritability measures how much life span can be explained by genetic differences, excluding differences such as lifestyle, sociocultural factors, and accidents.
While previous estimates of heritability in the human life span ranged from 15 to 30%, in the new study it was probably no more than seven percent, maybe even lower.
For this study, which was published in the journal Genetics, the team used online genealogy sources with public family trees produced by customers representing six billion ancestors.
Removing excessive entries and from people who are still alive, they sew the remains of the genealogies together including more than 400 million people, mostly Americans of European descent.
Each of them is connected to the other both by parent-child relationships or partners.
They focused on relatives born in the 19th and early 20th centuries, and noted that couples' life spans tended to be correlated, more similar than siblings of the opposite sex.
Comparing different types of in-laws, they found that first-in-law brothers and sisters-in-law correlated life span, even though there were no blood relations and generally did not share households.
The finding that the siblings of one partner's spouse or their spouse had the same life span as their own made it clear that something else was playing, the researchers said.
The answer might lie in assortative marriage. People tend to choose partners with traits like theirs – in this case, how long they live, they explain.