In Paris in 1876 a 31-year-old banker named Albert took an 18-year-old named Bettina as his wife. Both were Rothschilds, and they were cousins. According to conventional notions about inbreeding, their marriage ought to have been a prescription for infertility and enfeeblement. In fact, Albert and Bettina went on to produce seven children, and six of them lived to be adults. Moreover, for generations the Rothschildfamily had been inbreeding almost as intensively as European royalty, without apparent ill effect. Despite his own limited gene pool, Albert, for instance, was an outdoorsman and the seventh person ever to climb the Matterhorn. The American du Ponts practiced the same strategy of cousin marriage for a century. Charles Darwin, the grandchild of first cousins, married a first cousin. So did Albert Einstein. In our lore, cousin marriages are unnatural, the province of hillbillies and swamp rats, not Rothschilds and Darwins. In the United States they are deemed such a threat to mental health that 31 states have outlawed first-cousin marriages. This phobia is distinctly American, a heritage of early evolutionists with misguided notions about the upward march of human societies. Their fear was that cousin marriages would cause us to breed our way back to frontier savagery—or worse. "You can't marry your first cousin," a character declares in the 1982 play Brighton Beach Memoirs. "You get babies with nine heads." So when a team of scientists led by Robin L. Bennett, a genetic counselor at the University of Washington and the president of the National Society of Genetic Counselors, announced that cousin marriages are not significantly riskier than any other marriage, it made the front page of The New York Times. The study, published in the Journal of Genetic Counseling last year, determined that children of first cousins face about a 2 to 3 percent higher risk of birth defects than the population at large. To put it another way, first-cousin marriages entail roughly the same increased risk of abnormality that a woman undertakes when she gives birth at 41 rather than at 30. Banning cousin marriages makes about as much sense, critics argue, as trying to ban childbearing by older women. But the nature of cousin marriage is far more surprising than recent publicity has suggested. A closer look reveals that moderate inbreeding has always been the rule, not the exception, for humans. Inbreeding is also commonplace in the natural world, and contrary to our expectations, some biologists argue that this can be a very good thing. It depends in part on the degree of inbreeding.

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. 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. So how do scientists reconcile the experience in Bradford with the relatively moderate level of risk reported in the Journal of Genetic Counseling? How did Rothschilds or Darwins manage to marry their cousins with apparent impunity? Above all, how could any such marriages ever possibly be beneficial? The traditional view of human inbreeding was that we did it, in essence, because we could not get the car on Saturday night. 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 percent of all marriages in history have been between second cousins or closer.

Can you marry a cousin? Laws governing the marriage of first cousins vary widely. In 24 states (pink), such marriages are illegal. In 19 states (green), first cousins are permitted to wed. Seven states (peach) allow first-cousin marriage but with conditions. Maine, for instance, requires genetic counseling; some states say yes only if one partner is sterile. North Carolina prohibits marriage only for double first cousins. Got that?

Map by Matt Zang Source: cousincouples.com and Cuddle International.

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. And for their descendants? How could the remarkably untroubled reproductive experience of intermarried Rothschilds differ so strikingly from that of intermarried families in Bradford? 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. A founding couple 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. 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.

Global Inbreeding 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.

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.

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The Inbred Rothschild Family This picture gallery portrays members of five generations of the legendary Rothschild banking family, beginning with founder Mayer Amschel and his wife, Gutle. In an effort to build the fortune he had created, Mayer wrote a will that made intermarriage lucrative for his offspring. They took his point and frequently inbred: Cousins began marrying cousins, and in one case, a niece wed her uncle. Albert considered marrying only two women, both cousins. He chose Bettina, with whom he had seven children. Subsequent generations began to outbreed more frequently.

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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.