Previous studies have reported that related human couples tend to produce more children than unrelated couples but have been unable to determine whether this difference is biological or stems from socioeconomic variables. Our results, drawn from all known couples of the Icelandic population born between 1800 and 1965, show a significant positive association between kinship and fertility, with the greatest reproductive success observed for couples related at the level of third and fourth cousins. Owing to the relative socioeconomic homogeneity of Icelanders, and the observation of highly significant differences in the fertility of couples separated by very fine intervals of kinship, we conclude that this association is likely to have a biological basis.
What’s going here? p-ter and John Hawks have some thoughts. I can conceive of three broad classes of reasons for this dynamic: physiological, bio-behavioral and social. In the first class there are concerns about mother-fetus immune conflict, which might be more likely when parents are genetically very distant. A result of this would be increased miscarriage rates among these highly exogamous pairings. In the second class there are a range of possibilities; for example, the MHC profile mix makes third and fourth cousins more “sexy” to each other so that they simply have intercourse more often. The third option is that more closely related individuals have more in common, can pool resources across families more efficiently, and so forth. The sum of these factors may make these partnerships more economically successful, and this naturally entails more offspring. Above I’ve taken figure 1C from the paper, which plots the number of grandchildren as a function of relatedness (starting with second cousins to the far left). I’ve editing the figure to include in the coefficient of relationship assuming no more complex inbreeding (e.g., double cousins and so on). I just wanted to illustrate how relatedness converges upon 0; this is probably an overestimate of the drop in relatedness because in a finite population lineages tend to be reticulated so that some individuals will show up repeatedly in your ancestry tree rather soon as you move up the tree. But the point is that I don’t see how an r of 1/512 is really that different from an r of 1/2048 to make such a big impact in terms of reproductive fitness. The deleterious recessive effects which reduce fitness of first cousins drops off really quickly, and I don’t see why the benefits of MHC compatibility, both physiological and behavioral, would persist and then drop so far out on the spectrum of relatedness.
So here’s a model: perhaps this dynamic is a function of the difference between genetic variance and cultural variance, and the reproductive sweep spot is the region where the former is maximized across the reproductive pair and the latter minimized. What do I mean by this? Genetic differences across populations are often clinal; that means drawing population boundaries can be difficult. Gene flow across demes quickly equilibrates allele frequencies so that drift can’t fix differences. But culture may be different; whereas it is very difficult to maintain greater between group variance than within group variance with genetics for coterminous populations, it is not as difficult when it comes to culture. For example, the language of Greeks and Turks differs a great deal more than their genetics. Greeks are linguistically closer to the English or Punjabis than they are to the Turks; but genetically they are far closer to Turks than these other groups (until ~1920 they resided across much of the same geographic expanse). Perhaps in Iceland during this period the differences in mores in family groups exhibited discontinuities. Third or fourth cousins might have been within the arc of these clusters of mores, and so have more in common in terms of shared values and outlooks. As one moved beyond this limit one was more likely to cross the boundary which separated local subcultures, and so less amity might have resulted from these marriages. The increase in reproductive fitness up to third and fourth cousins is simply a function of the biological parameters; e.g., deleterious recessive diseases. But at this point biology is far less important than culture. Third and fourth cousins lie beyond curtain of biological risk, but behind the wall of cultural unfamiliarity. The sweet spot.
This is what I mean as an image: