The idea that inbreeding might sometimes be beneficial is clearly contrarian. So it's important to acknowledge first that inbreeding can sometimes also go horribly wrong—and in ways that, at first glance, make our stereotypes about cousin marriage seem completely correct.

In the Yorkshire city of Bradford, in England, for instance, a majority of the large Pakistani community can trace their origins to the village of Mirpur in Kashmir, which was inundated by a new dam in the 1960s.
Cousin marriages have been customary in Kashmir for generations, and more than 85 percent of Bradford's Pakistanis marry their cousins.
Local doctors are seeing sharp spikes in the number of children with serious genetic disabilities, and each case is its own poignant tragedy.
One couple was recently raising two apparently healthy children. Then, when they were 5 and 7, both were diagnosed with neural degenerative disease in the same week. The children are now slowly dying.
Neural degenerative diseases are eight times more common in Bradford than in the rest of the United Kingdom.
Another family that practiced inbreeding was the du Pont family of Delaware. The Rothschilds and the duPonts primarily wanted to keep the wealth within their family and inbreeding seemed the best way to do that. But the duPonts were not as lucky as the Rothschilds had been. ,.
The duPont family, who had run the business personally for generations prior to becoming a corporate entity, had some serious genetic problems. Many of the family members ended up with the worst traits of inbreeding from physical disfigurement to raving insanity.
One duPont family member even went as far as killing an olympic wrestler that he was supporting out of the clear blue sky.
This history of mental disturbance has followed the corp like a foul cloud, tainting many of the operations that the duPont family became involved in. The last duPont died in 2003.
-----You kind of have to feel sorry for the original duPonts. They took their whole family line and ran it into the ground.

The great hazard of inbreeding is that it can result in the unmasking of deleterious recessives, to use the clinical language of geneticists. Each of us carries an unknown number of genes—an individual typically has between five and seven capable of killing our children or grandchildren. These so-called lethal recessives are associated with diseases like cystic fibrosis and sickle-cell anemia.

Most lethal genes never get expressed unless we inherit the recessive form of the gene from both our mother and father. But when both parents come from the same gene pool, their children are more likely to inherit two recessives.
Perhaps one of the more infamous examples of a recessive gene popping up because of inbreeding is in the case of Queen Victoria, who carried a recessive gene for hemophilia, hers probably the result of a mutation.
One of her sons was a hemophiliac and two of her daughters carried the recessive trait. When she married her children off to royal families throughout Europe, she spread the gene throughout the royal houses of the continent. As the royals continued inbreeding, the disease was the scourge and fear of royalty for several generations.
Though there have been no cases of hemophilia in the last two generations of Victoria's family, it could yet reappear, as the gene can remain hidden for as long as six generations.
The consequences of inbreeding are unpredictable and depend largely on what biologists call the founder effect: If the founding couple pass on a large number of lethal recessives, as appears to have happened in Bradford, these recessives will spread and double up through intermarriage.
If, however, Mayer and Gutle Rothschild handed down a comparatively healthy genome, their descendants could safely intermarry for generations—at least until small deleterious effects inevitably began to pile up and produce inbreeding depression, a long-term decline in the well-being of a family or a species.

So how do scientists reconcile the experience in Bradford and others with the relatively moderate level of risk reported in the Journal of Genetic Counseling?
Above all, how could any such marriages ever possibly be beneficial?

The Conclusion tomorrow on how inbreeding can and has been successful in the past.