Contrary to the popular belief in America that very few people actually marry a second cousin or closer, the truth of the matter is that with almost certainty we all have ancestors who inbred--and not that far back in our gene pool!

Until the past century, families tended to remain in the same area for generations, and men typically went courting no more than about five miles from home—the distance they could walk out and back on their day off from work. As a result, according to Robin Fox, a professor of anthropology at Rutgers University, it's likely that 80 per cent of all marriages prior to the 20th Century were between second cousins or closer.

 

Researchers who study inbreeding track consanguineous marriages—those between second cousins or closer. In green countries, at least 20 percent and, in some cases, more than 50 percent of marriages fall into this category. Pink countries report 1 to 10 percent consanguinity; peach-colored countries, less than 1 percent. Data is unavailable for white countries. 
Map by Matt Zang
Map reproduced with the permission of A.H. Bittles

 Factors other than mere proximity can make inbreeding attractive. Pierre-Samuel du Pont, founder of an American dynasty that believed in inbreeding, hinted at these factors when he told his family: "The marriages that I should prefer for our colony would be between the cousins.

In that way we should be sure of honesty of soul and purity of blood." He got his wish, with seven cousin marriages in the family during the 19th century.

Mayer Amschel Rothschild, founder of the banking family, likewise arranged his affairs so that cousin marriages among his descendants were inevitable. His will barred female descendants from any direct inheritance.

Without an inheritance, female Rothschilds had few possible marriage partners of the same religion and suitable economic and social stature—except other Rothschilds. Rothschild brides bound the family together. Four of Mayer's granddaughters married grandsons, and one married her uncle. These were hardly people whose mate choice was limited by the distance they could walk on their day off. 

Some families have traditionally chosen inbreeding as the best strategy for success because it offers at least three highly practical benefits. First, such marriages make it likelier that a shared set of cultural values will pass down intact to the children. 

 Second, cousin marriages make it more likely that spouses will be compatible, particularly in an alien environment. Such marriages may be even more attractive for Pakistanis in Bradford, England, than back home in Kashmir.

 Intermarriage decreases the divorce rate and enhances the independence of wives, who retain the support of familiar friends and relatives. Among the 19th-century du Ponts, for instance, women had an equal vote with men in family meetings.

 Finally, marrying cousins minimizes the need to break up family wealth from one generation to the next. The rich have frequently chosen inbreeding as a means to keep estates intact and consolidate power.

Moderate inbreeding may also produce biological benefits. Contrary to lore, cousin marriages may do even better than ordinary marriages by the standard Darwinian measure of success, which is reproduction. A 1960 study of first-cousin marriages in 19th-century England done by C. D. Darlington, a geneticist at Oxford University, found that inbred couples produced twice as many great-grandchildren as did their outbred counterparts. 

Consider, for example, the marriage of Albert and Bettina Rothschild. Their children were descended from a genetic pool of just 24 people (beginning with family founders Mayer Amschel and Gutle Rothschild), and more than three-fifths of them were born Rothschilds.

In a family that had not inbred, the same children would have 38 ancestors. Because of inbreeding, they were directly descended no fewer than six times each from Mayer and Gutle Rothschild. If our subconscious Darwinian agenda is to get as much of our genome as possible into future generations, then inbreeding clearly provided a genetic benefit for Mayer and Gutle.

Just as a  founding couple can pass on disadvantageous genes, it can also pass on advantageous genes. Among animal populations, generations of inbreeding frequently lead to the development of coadapted gene complexes, suites of genetic traits that tend to be inherited together. These traits may confer special adaptations to a local environment, like resistance to disease.

 The evidence for such benefits in humans is slim, perhaps in part because any genetic advantages conferred by inbreeding may be too small or too gradual to detect.

Alan Bittles, a professor of human biology at Edith Cowan University in Australia, points out that there's a dearth of data on the subject of genetic disadvantages too. Not until some rare disorder crops up in a place like Bradford do doctors even notice intermarriage. 

So, while we have lots of data on the disadvantages of inbreeding, we have very little on the advantages. 

 Something disturbingly eugenic about the idea of better-families-through-inbreeding also causes researchers to look away. Oxford historian Niall Ferguson, author of The House of Rothschild, speculates that that there may have been "a Rothschild 'gene for financial acumen,' which intermarriage somehow helped to perpetuate. Perhaps it was that which made the Rothschilds truly exceptional." But he quickly dismisses this as "unlikely."

 At the same time, humans are perfectly comfortable with the idea that inbreeding can produce genetic benefits for domesticated animals. When we want a dog with the points to take Best in Show at Madison Square Garden, we often get it by taking individuals displaying the desired traits and "breeding them back" with their close kin. 

 Researchers have observed that animals in the wild may also attain genetic benefits from inbreeding. Ten mouse colonies may set up housekeeping in a field but remain separate.

 

The dominant male in each colony typically inbreeds with his kin. His genes rapidly spread through the colony—the founder effect again—and each colony thus becomes a little different from the others, with double recessives proliferating for both good and ill effects. When the weather changes or some deadly virus blows through, one colony may end up better adapted to the new circumstances than the other nine, which die out. 

 Inbreeding may help explain why insects can develop resistance almost overnight to pesticides like DDT: The resistance first shows up as a recessive trait in one obscure family line. Inbreeding, with its cascade of double recessives, causes the trait to be expressed in every generation of this family—and under the intense selective pressure of DDT, this family of resistant insects survives and proliferates. 

Click on the image to enlargeÂ

 

The obvious problem with this contrarian argument is that so many animals seem to go out of their way to avoid inbreeding. Field biologists have often observed that animals reared together from an early age become imprinted on one another and lack mutual sexual interest as adults; they have an innate aversion to homegrown romance. 

 But what they are avoiding, according to William Shields, a biologist at the State University of New York College of Environmental Science and Forestry at Syracuse, is merely incest, the most extreme form of inbreeding, not inbreeding itself.

He argues that normal patterns of dispersal actually encourage inbreeding. When young birds leave the nest, for instance, they typically move four or five home ranges away, not 10 or 100; that is, they stay within breeding distance of their cousins.

Intense loyalty to a home territory helps keep a population healthy, according to Shields, because it encourages "optimal inbreeding." This elusive ideal is the point at which a population gets the benefit of adaptations to local habitat—the coadapted gene complexes—without the hazardous unmasking of recessive disorders.
 

In some cases, outbreeding can be the real hazard. A study conducted by E. L. Brannon, an ecologist at the University of Idaho, looked at two separate populations of sockeye salmon, one breeding where a river entered a lake, the other where it exited.

Salmon fry at the inlet evolved to swim downstream to the lake. The ones at the outlet evolved to swim upstream. When researchers crossed the populations, they ended up with salmon young too confused to know which way to go. In the wild, such a hybrid population might lose half or more of its fry and soon vanish.

  It is, of course, a long way from sockeye salmon and inbred insects to human mating behavior. But Patrick Bateson, a professor of ethology at Cambridge University, argues that outbreeding has at times been hazardous for humans too

. For instance, the size and shape of our teeth is a strongly inherited trait. So is jaw size and shape. But the two traits aren't inherited together.

If a woman with small jaws and small teeth marries a man with big jaws and big teeth, their grandchildren may end up with a mouthful of gnashers in a Tinkertoy jaw.

Before dentistry was commonplace, Bateson adds, "ill-fitting teeth were probably a serious cause of mortality because it increased the likelihood of abscesses in the mouth." Marrying a cousin was one way to avoid a potentially lethal mismatch.

 Bateson suggests that while youngsters imprinting on their siblings lose sexual interest in one another,  they may also gain a search image for a mate—someone who's not a sibling but like a sibling. Studies have shown that people overwhelmingly choose spouses similar to themselves, a phenomenon called assortative mating. The similarities are social, psychological, and physical, even down to traits like earlobe length. Cousins, Bateson says, perfectly fit this human preference for "slight novelty." 

  So where does this leave us? No scientist is advocating intermarriage, but the evidence indicates that we should at least moderate our automatic disdain for it. One unlucky woman, whom Robin Bennett encountered in the course of her research, recalled the reaction when she became pregnant after living with her first cousin for two years.

 Her gynecologist professed horror, told her the baby "would be sick all the time," and advised her to have an abortion. Her boyfriend's mother, who was also her aunt, "went nuts, saying that our baby would be retarded." The woman had an abortion, which she now calls "the worst mistake of my life."

 Science is increasingly able to help such people look at their own choices more objectively. Genetic and metabolic tests can now screen for about 100 recessive disorders.

In the past, families in Bradford rarely recognized genetic origins of causes of death or patterns of abnormality. The likelihood of stigma within the community or racism from without also made people reluctant to discuss such problems.

But new tests have helped change that. Last year two siblings in Bradford were hoping to intermarry their children despite a family history of thalassemia, a recessive blood disorder that is frequently fatal before the age of 30. After testing determined which of the children carried the thalassemia gene, the families were able to arrange a pair of carrier-to-noncarrier first-cousin marriages.

 Such planning may seem complicated. It may even be the sort of thing that causes Americans, with their entrenched dread of inbreeding, to shudder. But the needs of both culture and medicine were satisfied, and an observer could only conclude that the urge to marry cousins must be more powerful, and more deeply rooted, than we yet understand. 



Web sites devoted to the topic of consanguinity and cousin marriages abound, with approaches ranging from academic to activist: www.consang.net, www.cousincouples.com, and www.cuddleinternational.org

Discovermagazine.com/2003/aug/featkiss