The Guardian is running an essay by two British GP's about the consequences of cousin marriage in the UK: "Cousin marriages: a question of understanding".
The fact remains that this is a health issue, which has lead to a higher than normal infant and perinatal mortality in cities such as Birmingham and Bradford. The extent of the morbidity is not yet known, but a fair indication can be found in the rise in the number of applications for disability living allowances for children in these areas. A disproportionate number of cases occur among those of south Asian, and especially Pakistani descent, but it can affect all ethnicities and should be a concern for all. It is time to discuss it rationally so we can enable informed reproductive choices.
The topic of cousin marriage highlights the cultural tensions with immigrant populations in Europe, the demographic impacts of immigration (where the pool of culturally preferred partners is not as large as in the nation of origin), and the economics of integrating high-fertility immigrant populations into national health programs. From the evolutionary perspective, it highlights the importance of founder effects and demography in determining disease risk.
It strikes me as a case where genetic testing might provide positive benefits in human and economic terms, yet not without cost to cultural preferences and government ambitions. Legal prohibitions on cousin marriage reflected first and foremost dynastic concerns (limiting the concentration of power in aristocratic families), but later mostly eugenic concerns. The mating behavior of aristocrats is no longer a serious issue for most of us. If genetic testing can substantially reduce the risk of autosomal recessive disorders for the offspring of cousins, doesn't that negate the eugenic argument?
The authors mention genetic testing only briefly, mentioning that it cannot eliminate all risk of all rare autosomal recessive disorders because we cannot test for every possible disease-associated variant. That is true, but at some point the effectiveness of screening for known variants makes the additional risk of cousin marriage very slight compared to the ordinary risk in outbred marriages.
What remains is the question of inbreeding depression. How important are genes of small effect to phenotypes we care about? If they're very important, then health-related phenotypes may be suboptimal in offspring of cousin marriages even if no Mendelian disorder is present. This would be a broad eugenic argument for banning cousin marriage, even with the availability of testing.
How far can this eugenic argument go? Maybe public health advocates should cast their net more widely, encouraging outbreeding on a broader scale than immigrant communities. You see how these topics are intertwined -- eugenics, genetic testing, and State regulation of marriage. The GP authors of the Guardian piece make all the standard points, which amount to "Won't anyone think of the children!?" But where does this turn into explicitly eugenic arguments, such as Oliver Wendell Holmes' famous, "Three generations of imbeciles are enough"?
I'm inclined toward the opinion that people should choose themselves, and technology should facilitate their choices, not foreclose them. The State may educate, it shouldn't dictate. But not everyone agrees, and most argue for some point at which the State should bar people from destructive choices. But where is that, and who should control the information? That's what's at stake in genetic testing.