An Association Between Kinship And Fertility of Human Couples. Agnar Heigason, Snaebjoern Palsson, Daniel F. Guobjartsson, Pordur Kristjansson and Karl Stefanson, SCIENCE vol 329 8 February 2008 page 813 figure 3 C.
Grandchildren in the Iceland study:
Quite helpfully, the Iceland study includes not only the comparison between relatedness and the number of offspring, but some other information including the comparison between the relatedness of a couple and the number of grandchildren. Here is what they found. The vertical axis is a measure of the number of grandchildren, and the horizontal axis is the degree of relatedness, decreasing toward the left.
An Association Between Kinship And Fertility of Human Couples. Agnar Heigason, Snaebjoern Palsson, Daniel F. Guobjartsson, Pordur Kristjansson and Karl Stefanson, SCIENCE vol 329 8 February 2008 page 813 figure 3 C.
The first interval is second cousins or closer, then third cousins or closer, fourth cousins or closer, fifth cousins or closer, sixth cousins or closer, seventh cousins or closer and eighth cousins or closer or no known relationship.
The study takes the long view. They go forward two generations and back nine. The fact that they did not go beyond eighth cousins suggests that there are no or very few children at ten generations separation. That is consistent with our usual finding of ten generations before a civilization crashes. Taking the long view is the right approach. Mating decisions made now affect things generations later.
The curve again closely resembles the curve generated by the model. But we do not have to wave our hands and talk about first cousin results when there are none. We already see the effect of too much relatedness at second cousin and closer.
The best degree of relatedness for maximum number of grandchildren in this very homogeneous population is around third or fourth cousin. Fifth cousins are also a good match. There are fewer grandchildren but possibly greater stability. By sixth cousin, the number of grandchildren looks like it just barely reaches the replacement level. Past that, there is a problem.
It still remains to be seen what the results might be in a less homogenous population, but it seems impossible that the curve would shift to the right. Almost surely the curve shifts to the left. Just how far it shifts is a question that will probably devil us for a long time.
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