LONGEVITY: Genetics play less of a role in lifespan than we thought.

The calculations are complicated, but the thinking behind them is simple: when genes play a role in a trait, you should be pretty similar to your siblings and parents, a little less similar to your cousins, less similar to your second cousins, and so on. Ruby and his colleagues used the family tree data to explore whether lifespans were similar between more closely and distantly related relatives. This produced heritability estimates similar to those calculated before: siblings’ lifespans were highly correlated, first cousins’ a bit less correlated, and so on.

But spouses’ lifespans were correlated, too. That could be easily explained by spouses sharing the same household and lifestyle: eating the same healthy diet or puffing on cigarettes together. But the researchers noticed something odd: the lifespans of other relatives related only by marriage also correlated. That can’t be explained by genes, and it can’t be explained by shared environment.

So Ruby and his colleagues started investigating the lifespans of in-laws. They looked at siblings-in-law, and first-cousins-in-law, and then further afield, at relationships like “the sibling of a sibling’s spouse” (your brother’s wife’s sister) and “the spouse of a spouse’s sibling” (your husband’s sister’s husband). Even at these distant relationships, lifespans were correlated—if your spouse’s sibling’s spouse lived to a ripe old age, that means you’re a bit more likely to do the same.

What’s going on here is assortative mating: people are likely to marry people who match them on certain characteristics, like education and wealth, and those traits are in turn related to longevity.

I’d wager that happiness plays a factor in this as well.