These records kept detailed track of not only the marriages, births, and deaths, but also of the harvest each year. Being an impoverished and remote place, the residents of Överkalix starved when the harvest failed. Conversely, years of great bounty allowed Överkalixians to eat very well indeed.
Scientists found that boys who enjoyed those rare overabundant winters — kids who went from normal eating to gluttony in a single season — produced sons and grandsons who lived far shorter lives. Put simply, the data suggested that a single winter of overeating as a youngster could initiate a biological chain of events that would lead one's grandchildren to die decades earlier than their peers did.
The phenemona that causes this is "epigenetics"— changes in gene activity that do not involve alterations to the genetic code but still get passed down to at least one successive generation. I've drawn the above quotes from a fascinating TIME article from about two years ago. The YouTube video above is a BBC Horizon documentary on epigenetics, which I found just as interesting.
I find it really cool that the attention to detail by a little parish church in a cold corner of Sweden could lead to such scientific breakthroughs!