As Guiseppe Passarino of the University of Calabria in Rende, Italy, says in the New Scientist coming out this week:
“Everyone knows that inbreeding is bad – it increases your chances of catching a range of diseases,” he says. “But on the other hand, our study suggests that if inbreds don’t get those diseases when they’re young, they might have a better chance of long life.”
He has been studying centenarians in remote villages in Italy for years. He was part of the study that looked at genes that are more prevalent among old people, and found they were also prevalent among early deaths from disease (a.k.a. if you have the gene you're on a lottery ticket for an earlier or a much later death, see http://www.nature.com/ejhg/journal/v9/n9/abs/5200703a.html)
The study points out that "in a small, inbred community, the gene pool remains the same and it is more likely that an individual will be “homozygous” –with two identical copies of a gene. “Longevity seems to be linked to homozygosity,” Passarino says. This may be because certain copies of some genes boost lifespan, and carrying two of them doubles the effect. A number of DNA analyses have located regions of the genome where centenarians show an unusually high level of homozygosity, he says."
The bad news girls? Apparently this only really works for men. "Passarino says it may be because the genetic component of longevity plays a more important role in men, whereas in women, environmental factors come to the fore."For example, places that have better healthcare tend to dramatically improve women's longevity but less so men (in Denmark "the number of male centenarians is 10 times as high because of better healthcare, but the number of females is 50 times higher")
And of course this doesn't mean you should really be marrying your sister...
"Bruce Carnes of the University of Oklahoma cautions against marrying a relative, however. “Homozygosity is typically a very bad thing,” he says. “Almost every discussion of inbreeding that I have ever read has emphasised its downside.” You only have to look at the Spanish line of kings - http://en.wikipedia.org/wiki/Charles_II_of_Spain )
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