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While doing research today I run into this piece:

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.

After investigating further I found this response that extends and reinforces the finding using a different measure - martial radius:

Both studies corroborate the hypothesis that the superposed contrary forces of inbreeding and outbreeding depression have an effect on human fertility. The coincidence of the conclusions of these two independent large-scale studies suggests that this is a general phenomenon in human populations.

This response, however, calls the marital radius metric into question.

What are these studies representing? Does they argue acceptably for or against the case?

Oddthinking
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Ask4Knowledge
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    Please include relevant quotes in your question. There's no guarantee that those links won't break or that someone else won't be able to access them for one reason or another. – Rob Watts Feb 22 '15 at 03:46
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    [Welcome to Skeptics!](http://meta.skeptics.stackexchange.com/questions/1505/welcome-to-new-users) Typically, we address questions here by finding more reliable sources than the questioner used. In this case, however, you are citing from the scientific literature about an ongoing discussion. It is going to be very difficult to answer your question if the science is still being established. – Oddthinking Feb 22 '15 at 04:26
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    The papers are all from 2008, so possibly there's more recent evidence somewhere. – ChrisW Feb 22 '15 at 07:50
  • Well, the main interest for me would be validating these studies since the conclusion I arrived calls the results in the question. The correlation they found is moderate, but I would argue they would have needed higher to argue successfully for the case. If we look at the history of Iceland its seems like poor rural area which was largely effected with natural disasters and migration within the time the studied couples lived. So I would like to argue that with these conditions maybe marrying your distant relative was logical but not because of biology. And when Iceland modernized that ended. – Ask4Knowledge Feb 22 '15 at 10:34
  • And since the association weakens after 1900 I would be very careful to make any conclusions from there results, Like I said earlier I would argue that if there is difference it could also be other way around since even after massive benefits there is no massive advantage at all. – Ask4Knowledge Feb 22 '15 at 10:53

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