Chris Patil notes a point I missed on recent research into human telomere lengths: "The paper's key finding (from my perspective, at least) didn't make it into the title: Telomere length is positively correlated with lifespan. While I'm aware of studies linking telomerase activity in animal species to lifespan, the growing consensus seems to be that this is a corollary of the relationship between telomerase and body mass, which in turn is correlated with lifespan; furthermore, the relationship is the opposite of the one here (larger, longer-lived animals tend to have lower telomerase activity than smaller, shorter-lived animals with a similar body plan; see our earlier article Telomerase correlates negatively with body mass). This is the first study I'm aware of in which human-to-human variation in telomere length (which occupies a concept space somewhere between the strictly genetic and the strictly epigenetic) has been shown to correlate with lifespan. The key issue now, of course: Where are the causal links, if any, between the telomeres a person inherits and their life expectancy?" This will no doubt boost the fundraising ability of companies presently building ways to manipulate telomere length.