"Dr. Stefansson suggested what we might call a Goldilocks Zone for inbreeding. That term, which we usually see applied to exoplanets, refers to the idea that planets need to neither too far away from nor too close to their star in order to be able to support life. In much the same way, third cousins might actually have just the right amount of genetic overlap, neither too similar nor too dissimilar, and so they enjoy a reproductive advantage. However, the underlying genetics of that explanation remain unknown."
io9.com/5863666/why-inbreeding-really-isnt-as-bad-as-you-think-it-is