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At the next family reunion

Couples who are third or fourth cousins tend to have more kids and grandkids than other couples. And though considered somewhat of a cultural taboo, mating between "kissing cousins" makes good biological sense, say scientists.

The findings, which come from a recent study of Icelanders, shed light on how relatedness affects reproduction and ultimately the size of families.

The researchers suggest marrying third and fourth cousins is so optimal for reproduction because they sort of have the "best of both worlds." While first-cousin couples could have inbreeding problems, couples who are far-removed from each other could have genetic incompatibilities.