A long life tends to run in the family tree. But even though genetics certainly has a role to play in our longevity, it isn't as important as we've assumed until now.
An analysis of over 400 million people has revealed the genetic influence on life span is well below past estimates. Instead, the study suggests there's something else at work - something more than just inheritance or a shared environment.
That something is choice. By choosing partners that look and act similarly to us, we are passing on cultural and genetic factors that are linked to life span - kind of like how we pass on our wealth or socioeconomic status to the next generation.
"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," says research lead Graham Ruby, who studies the ageing process in humans at the company Calico Life Sciences.
"It helps contextualize the questions that scientists studying aging can effectively ask."
When figuring out how certain traits are passed on, scientists use a tool of measurement called heritability. It estimates the degree of variation in a trait - such as one's life span - by comparing the effects of genetic differences, as opposed to non-genetic differences like lifestyle, sociocultural factors, and accidents.
Previous studies have placed the heritability of human life span somewhere around 15 to 30 percent. But using the online genealogy resource Ancestry, the new study suggests this estimate is far too high.
"Partnering with Ancestry allowed this new study to gain deeper insights by using a much larger data set than any previous studies of longevity," says Ball.
Sifting through that data, the researchers picked out a set of family trees encompassing over 400 million people born across the 19th and 20th centuries - each of whom were connected to each other by a parent-child or spouse-spouse relationship.
Creating a model out of the data, the researchers noticed something unusual that hadn't been found in previous studies.
It appeared that the life spans of spouses were unusually similar - more similar, in fact, than in siblings of opposite gender who share far more genes.
This could be do to the fact that spouses share the same environment, but there was a clue in the data that suggested something else was going on.
Even though they are not blood relatives and do not generally share households, it was found that siblings-in-law and first-cousins-in-law had similar life spans, too.
When the researchers began looking at other distant in-law relationships, like uncles, aunts and first cousins, they noticed the same pattern: both sides of the family tree had similar life spans.
Above and beyond genetics and shared households, the data suggests there is a third factor at play, and it's called assortative mating. This is the phenomenon whereby people tend to select partners with traits similar to their own.
"What assortative mating means here is that the factors that are important for life span tend to be very similar between mates," says Ruby.
This doesn't mean that humans have some sixth sense that allows them to see in advance how long their partners are going to live.
Instead, the authors say humans are most likely selecting partners with similar characteristics, both genetic and sociocultural, that are linked to longevity of life.
For instance, if wealth leads to a longer life, and people tend to marry those that are equally wealthy, then this would lead to similar life spans among families with similar wealth.
This also goes for characteristics that are determined by genetics. So if tall people prefer tall spouses, and height determines how long you live, the same pattern would be found.
By taking into account assortative mating, the researchers found that life span heritability is no more than seven percent, maybe even lower.
This is half as much as previous estimates, and this means that a long life may have far less to do with our genes than we once thought.
The study has been published in Genetics.