According to "An Association Between the Kinship and Fertility of Human Couple" (http://science.sciencemag.org/content/319/5864/813.long http://science.sciencem

According to "An Association Between the Kinship and Fertility of Human Couple" (http://science.sciencemag.org/content/319/5864/813.long http://science.sciencemag.org/content/319/5864/813.long), third (or fourth) cousins have optimal reproductive success. Reproductive success is not just the number of children (because 1st or 2nd cousins couples tend to have more children but with associated health and/or reproductive problems), but the whole descendance.