A common opinion has always been this: Genetics are the main determinant of life span. So if you still want, go ahead and exercise. Go ahead and eat your kale. Do whatever those silly one-hundred-years-plus crones on the Today show recommend you do to live a long time, but none of it matters – your shelf life is largely predetermined.

To continue the analogy, you're not much different than a loaf of bread or a carton of cottage cheese because you too have an expiration date, only yours is written into your genetic code, courtesy of your parents. If they died in their seventies, so will you, providing the unexpected pandemic or falling piano doesn't take you out earlier.

C'mon, tell me the truth, you encounter that kind of thinking all the time, don't you? At least from people who are looking for an excuse not to exercise and to eat what their heart desires instead of what their heart requires.

Any maybe you, too, are occasionally prone to that kind of thinking. You exercise and you eat right and take life-extending supplements, but you secretly wonder how much good they are in thwarting the countdown clock that's part of your genetic code.

I got news for you. Genetics only plays a tiny role in determining how long you live. How tiny? Around 7 percent. At least that's what an analysis of a massive database of over 400 million records indicates.

Hey Sweet Cheeks, You Interested in a Little Assortative Mating?

Most previous scientific estimates on the heritability of human longevity have pegged it somewhere between 20 and 30 percent, which is still low enough to come as a surprise to most people. The way insurance agents, people who read the health section of the local newspaper, and grizzled old general practitioners talk, you'd think heredity plays a much bigger role in life span.

They, along with the rest of us, can probably be forgiven for not staying abreast of the thrilling world of statistical genetics. But forget all that, the 7 percent cited in the study I alluded to is much lower than anyone thought, including geneticists.

Why the big difference between this study and previous studies that showed genetics to play a much bigger role in how long you live? Two reasons.

First, this analysis, conducted by Calico Life Sciences, tapped into databases from Ancestry, the genealogy and consumer genomics company. That gave them access to data of family trees including more than 400 million individuals. No other study has had access to a database that large.

Secondly, this study took into consideration the well-known genetic aspect of "assortative mating," which is a genetics-speak for how people often choose mates based on things like shared religious, cultural, or ethnic interests; similar professional interests; or similar physical traits. You know, like seeks like.

However, they first looked for similarities in life span within families as opposed to the population at whole. When looked at in this way, their estimates of the role of heredity in longevity matched previous findings: somewhere between 20 and 30 percent.

But then they noticed something weird. Spouses had more similar life spans than those of opposite gender brothers and sisters. They then looked at in-laws who aren't genetically related and who don't share environments. They found a larger-than-expected connection between the life span of siblings-in-law and that of cousins-in-law. This correlation even extended to even more distant relationships, like the brothers and sisters of a sibling's spouse.

All of these correlations are consistent with assortative mating and once the researchers adjusted for it, they estimated the heritability of life span to be around 7 percent, well below previous estimates.

So What the Eff Does This Mean?

People of course can't/don't choose mates based on their potential life span. Instead, they often choose mates based on shared traits listed above (similar interests, religious beliefs, or physical features). However, they're also likely to choose mates based on similar socioeconomic status, which entails things like wealth and educational level. (People who share similar socioeconomic statuses are also more likely to meet each other.)

If the wealth and educational level is sufficient, those people, almost by default, have better sanitation and access to clean food and adequate healthcare. They possess the knowledge and means to eat well and incorporate exercise and supplements into their lifestyle.

So when such people choose "like" mates, they're more likely to live longer. They're also more likely to pass on their socioeconomic advantages to their children, thus rewarding them with longer lives.

All of this contributes to the observed effect of assortative mating and is seemingly responsible for muddling up all those previous studies and inflating the role heredity plays in longevity.

So sure, heredity plays a role in determining how long you live, but in most cases it's a bit role and not a starring role.

We're of course not talking about inheritable diseases or propensities for developing heart disease or certain types of cancer. Those things can certainly be inheritable and they can of course play a role in your life span, but when it comes to heredity, it doesn't play as big a role in senescence and entropy – you know, aging and lifespan – as we thought.

So it seems you're more the captain of your longevity ship than you or anyone else might have previously thought, so if you want to live long, continue filling your stowage with good, nutritious food and trimming your sails with exercise.

Reference

1. Graham Ruby J et al. Estimates of Heritability Are Substantially Inflated due to Assortative Mating. Genetics. 2018 Nov;210(3):1109-1124. PubMed.