Calico Life Sciences and Ancestry teamed up to use publicly available pedigree data to approach the problem of figuring out the genetic contributions to human longevity. The heritability of life span has been well investigated in the literature, with previous estimates ranging around 15-30%. But some of these studies found that it wasn't just blood relatives who shared similar life spans - so did spouses. This suggested that the heritability estimates might have been confounded by shared environments or assortative mating (the tendency to choose mates who have similar traits to ourselves). Starting from 54 million subscriber-generated public family trees representing six billion ancestors, Ancestry removed redundant entries and those from people who were still living, stitching the remaining pedigrees together.
The data set, called the SAP for "set of aggregated and anonymized pedigrees," included almost 500 million individuals (with a single pedigree accounting for over 400 million people), largely Americans of European descent, each connected to another by either a parent-child or a spouse-spouse relationship. The scale of the data allowed the researchers to get accurate heritability estimates across different contexts; they could stratify the data by birth cohort or by sex or by other variables without losing the power needed for their analyses.
Running the numbers, the team initially found heritability estimates to be between 15-30% - similar to the reported literature. But genetics aren't the only thing that can be passed down between generations: sociocultural factors can also influence certain traits, and these too can be inherited. The combination of genetic heritability and sociocultural heritability is the total transferred variance, that is, the total amount of variability in a trait that can be explained by inheritance. Researchers looked not only at siblings-in-law and first cousins-in-law but also examined correlation in both types of co-siblings-in-law. None of these relationship types generally share household environments, and yet their life spans showed correlation.
If they don't share genetic information and they don't share household environment, what accounts for the similarity in life span between individuals within these relationship types? Going back to their impressive dataset, the researchers were able to perform analyses that detected assortative mating. In other words, people tend to select partners with traits like their own - in this case, how long they live. Of course, you can't easily guess the longevity of a potential mate, but the basis of this mate choice could be genetic or sociocultural - or both. For a non-genetic example, if income influences life span, and wealthy people tend to marry other wealthy people, that would lead to correlated longevity. By correcting for these effects of assortative mating, the new analysis found life span heritability is likely no more than seven percent, perhaps even lower.
