A surprising new study combined family trees from over 400 million people and discovered that genes play a considerably smaller role than previously thought in how long people live.
The finding, published in the journal Genetics, comes from the largest investigation into the heritability of longevity conducted to date.
A team from US ageing research organisation Calico Life Sciences collaborated with Ancestry, an online genealogy company, to use publicly accessible family tree data.
The dataset, which lead author Graham Ruby says is nearly two orders of magnitude larger than data used in previous studies, enabled more nuanced analyses – including cousins-in-law, for example.
That’s how they discovered that “assortative mating” accounted for previously inflated estimates of lifespan heritability.
Assortative mating is a pattern in which people with similar traits, such as height, age or skin colour, for instance, mate with each other more often than would be expected by chance.
The researchers investigated family trees comprising mostly Americans of European descent born across a 120-year period through the nineteenth and early twentieth centuries, related via parent-child or spouse-spouse connections.
Lifespan heritability estimates for siblings and first cousins concurred in previous studies, which range between 20% to 30% for the same gender and under 15% for different genders.
As in such research, spousal life spans were also highly correlated in the latest findings, more so than siblings of opposite genders – but that could be due to shared environments. The researchers were able to tease this out by correlating lifespans of non-blood relatives – siblings-in-law and first cousins-in-law.
These associations were substantially higher than blood relatives, suggesting that rather than genes, assortative mating plays a major role in life span estimates.
In fact, genes predicted less than 10% of a person’s chances for a long life, and perhaps less than 7%.
“What is interesting and surprising,” says Ruby, “is that secondary assortative mating occurred because one cannot pick a mate based on longevity. Generally, people get married before either one of them has died.
“So, people select mates based on one set of factors, but a second factor – one that is not seen – in this case, longevity, becomes a secondary outcome.”
What does this mean for longevity research?
“We can potentially learn many things about the biology of ageing from human genetics, but if the heritability of lifespan is low, it tempers our expectations about what types of things we can learn and how easy it will be,” Ruby explains.
“Genes are far from the only factors that contribute to human life span, making the task of identifying a major gene involved in longevity more difficult.”
The findings support the contention, the authors suggest, that “sociocultural factors of relevance to human lifespan are indeed inherited through families”. That includes socioeconomic status, but also modifiable lifestyle choices such as diet and physical activity.
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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