At least, in Iceland. Jeanna Bryner's story tickles the "ick" factor and does a fairly good job of explaining the study by Anna Helgason and colleagues, reported in this week's Science:
The team found that women born between 1800 and 1824 and who partnered with a third cousin had an average of about four children and nine grandchildren, while those related to their mates as eighth cousins or more distantly had three children and seven grandchildren. A similar pattern showed up for women born between 1925 and 1949. Third cousins had an average of three children and about seven grandchildren, compared with two children and five grandchildren for eighth cousins and beyond.
I'd like to see how these data compare with other populations -- it seems to me that French Canadians probably have extensive enough records, and maybe Swedes. Not that there's any reason to disbelieve the results, just that it is very hard to correct for the social and cultural factors leading to marriages between distant relatives.
For one thing, people whose great-great-grandparents had lots of kids will obviously have more third cousins. To the extent that high-fertility great-great-grandparents are clumped together, it will be very hard to separate the effect of third cousin marriage from genetic heritability of fertility. As we all know, there has been lots of selection in recent human populations -- meaning that many alleles that influence fertility variance are segregating. So all things being equal, people who reproduce more are likely to be distant relatives.
There is a possible test -- just compare people who married third cousins with people who didn't marry their third cousins, but still had just as many third cousins. OK, so that might not be so easy, but if the demographic data are good enough it should be possible.
Another potential problem is outlined in the article: age at first birth:
One caveat: More closely related couples may just start making babies earlier than others. Past research has revealed "strong evidence that couples who were first cousins married earlier and were less likely to use contraception, the wives had their first child earlier, and they continued child-bearing at later ages," Bittles told LiveScience.
Yeah, that could be going on, too -- particularly if young, marriageable women are snatched up by distant relatives. Not too unlikely in a small town or farm community.
The paper gives some details that tend to blunt such criticisms. For one thing, it makes a significant difference whether the couple is sixth or seventh cousins. At that distance -- people who share a great-great-great-great-great grandparent -- the authors argue that any effect is most likely "biological" (read genetic), because other possible differences do not discriminate at that level.
In the end, the paper suggests that the demographic transition in modern urban societies is related to this kinship-mediated fertility effect:
The formation of densely populated urban regions that offer a large selection of distantly related potential spouses is a new situation for humans in evolutionary terms. We note that if the relationship between kinship and fertility has a basis in human reproductive biology, then it follows that the kind of demographic transition recently experienced by the Icelandic population could directly contribute to the slowing of population growth elsewhere through the relative increase of distantly related couples.
In other words, you thought that you were having fewer kids than your great-grandparents because you're living in a city instead of on a farm. But you're really having fewer kids because you're not married to your third cousin. Or something like that.
Helgason A, Pálsson S, Guðbjartsson DF, Kristjánsson Þ, Stefánsson K. 2008. An association between the kinship and fertility of human couples. Science 319:813-816. doi:10.1126/science.1150232