Habits of the World's Healthiest People

>"Science has determined that less than 25% of a person’s lifespan is determined by genes."

What does that mean? With a change of just 5% of a person's genes, they'd have been born a rat and died within a lifespan that differed from the average human by far more than just 5%. For that matter wheat shares about 50% of its genes with humanity.

I know it would be possible to make 5% alterations in one's food intake that would kill them instantly, so this isn't an argument for genetic determinism. The question is what does "X% of someone's Y comes from genes" mean when it's reported like this?