''You can't marry your first cousin,'' cautions one of the characters in Neil Simon's 1982 play Brighton Beach Memoirs. ''You get babies with nine heads.''

This is the classic folk-tale admonition against indulging in intra-familial relations that are a little too, well, intra; the resultant filmy-eyed, web-fingered, knuckle-dragging, tree- dwelling, leg-humping progeny are disturbingly familiar from gothic backwoods horror tales like Deliverance. They're the raving, drooling embodiment of the taboo of incest: an outrÃ© practice confined to swinging Egyptian pharaohs, obscure Inca tribes and the kind of patriarch whose parenting skills haven't evolved beyond imprisoning daughters in cellars and insisting on regular exercise of his 'visiting rights'. It's certainly not something condoned in polite society. Right?

Well, not quite. Many of the world's most prestigious families and a sizeable proportion of its royal dynasties have, over the centuries, flaunted more than their share of more- than-kissing cousins. They've not only cocked a snook at the increased risk of homozygosity - the chances of offspring being affected by recessive or deleterious genetic traits, magnified when both parents come from the same gene pool - but it's actually the preservation of those 'pure' genes that's been the point behind the whole family-that-lays-together-stays- together philosophy. Take the du Ponts, the family who founded the multinational company specialising in science-based solutions to... pretty much everything ('agriculture, nutrition, electronics, communications, safety and protection, home and construction, transportation and apparel,' according to its website). As Pierre Samuel du Pont, patriarch of the American clan, pronounced, back in the 19th century, '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 and then some, with seven inter-cousin marriages following over the next few decades. Du Pont-the-company, incidentally, went on to invent Teflon, perhaps in an effort to prove that even the most unsavoury of impressions needn't stick around forever.

Mayer Amschel Rothschild, founder of the banking family, also liked to keep things cosy, arranging his affairs so that cousin marriages among his descendants were inevitable. His will barred female descendants from any direct inheritance, so that female Rothschilds had a paucity of possible marriage partners of the same religion and suitable economic and social stature - except other Rothschilds. Thus, four of Mayer's granddaughters married his grandsons, and one married her uncle. In fact, between 1824 and 1877, of 36 male Rothschild descendants, 30 married their cousins, with first preferences going to those whose fathers were partners in different branches of the bank, giving a whole new dimension to the term 'family business'. Such homogeneity is as tight-knit as an XXXS sweater, and it's given rise to some disturbingly eugenic notions. Oxford historian Niall Ferguson, in his book, The House of Rothschild (Penguin), speculated that there might have been 'a Rothschild 'gene for financial acumen', which intermarriage somehow helped to perpetuate. Perhaps it was that which made the Rothschilds truly exceptional.' Though he later opted to hedge his bets, dismissing any putative dosh-gene as 'unlikely'.

Such obsessive-compulsive concern with 'pure' bloodlines would seem to reduce humans to the level of racehorses or Crufts contenders. But perhaps the most surprising thing is that the du Pont/Rothschild approach to genetic propagation seems to have been the historical rule rather than the exception. The traditional view of human inbreeding has been that, in the days when social mobility meant the wherewithal to make it to the inn of the hamlet on your own for the purposes of getting wasted, it was making a virtue of necessity; pre-internal combustion engine, families tended to remain in the same area for generations. 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 people who were second cousins or closer. In many ways, those marriages were the product of a defensive circle-of-wagons mentality; the rich would keep their estates and cultural values intact while consolidating power and wealth, and they would ensure that wives would retain the support of familiar friends and relatives (among the du Ponts, women had an equal vote with men in family meetings). In recognition of all this, marriages between first cousins were legalised in France and Italy in 1804, under the Napoleonic Code.



Three of history’s most high-powered in-breds… Charles II of Spain’s jaw was so deformed from inbreeding, he reportedly was unable to chew; Edgar Allen Poe married his cousin, Virginia Clemm, when she was 13; Charles Darwin was the grandson of first cousins, and married his own – he later carried out research into inbreeding in plants, motivated by how prone his his family was to disease and infertility.

In fact, it's been found that moderate inbreeding can produce, far from a super-abundance of crania, some biological benefits. Cousin marriages can do even better than outsourced marriages by the standard Darwinian measure of success, which is reproduction. A 1960 study of first-cousin marriages in 19th century England undertaken by C.D. Darlington, a geneticist at Oxford University, found that inbred couples produced twice as many great-grandchildren as did their outbred counterparts.Darwin himself had a keen interest in the intra-familial, for deeply personal as well as professional reasons; he was the grandson of first cousins, and had gone on to marry his own first cousin, Emma Wedgwood. He was responsible for some of the earliest research carried out on inbreeding, in plants, spurred on by worries about his own family's susceptibility to infectious disease and infertility, with three of his eight children dying of either tuberculosis or scarlet fever, and three of his remaining offspring's marriages childless. Darwin's son George went on to study cousin marriages, surveying mental hospital patients and finding that the percentage of those who were the issue of such unions was no greater than that in the general population but Darwin remained only partially convinced, having observed in his father's research cross- and inbreeding plants that the latter could have negative effects. (Other notables who've taken the do-you-take-this-cousin matrimonial route include Albert Einstein, H.G. Wells, Edgar Allen Poe and Jerry Lee Lewis, though the outcry over the latter two wasn't so much over their genetic intimacy as the fact that both their brides were 13 at the time of the unions in question.)Subsequent research has shown that the consequences of inbreeding are very much dependent on what biologists call the 'founder effect': if the founding couple pass on a large number of undesirable recessives, they will spread and enhance through intermarriage. If, however, the likes of Mayer Rothschild bequeathed a comparatively healthy genome, their descendants could safely intermarry for generations, until the small deleterious effects inevitably began to mount up and produce inbreeding depression: a long-term decline in the well-being and fitness of a family or species.

Or, particularly, a royal household. If the nobility liked to keep it in the family, then ruling dynasties were almost contractually obliged to paddle in the shallow end of the gene pool for diplomatic, political or strategic reasons, in order to expand, consolidate, or desperately cling on to their power. Medicis, Farneses, Romanovs and Bonapartes all inter-married furiously, but the champion shallow-enders were undoubtedly the Habsburgs, who ruled Spain from 1516 to 1700, and presided over a pan-European domain, with branches of the family reigning over Austria, Hungary, Belgium, the Netherlands and Germany. A genetic analysis of the family, carried out in 2009 at the University of Santiago de Compostela, found that nine out of 11 marriages during their 200-year dominion were between first cousins or uncles and nieces, and only half of the babies born to the dynasty in that period lived to see their first birthday, compared with about an 80 percent survival rate for children born in Spanish villages during the era.

The end of the genetic line was the unfortunate Charles II, born in 1661 and so homozygously challenged that his footprint was similar to that of the product of an incestuous union between a brother and sister or father and daughter. The portents were never stellar for Charles; his mother, Mariana of Austria, was a niece of his father, King Philip IV, and the daughter of Maria Anna of Spain and Emperor Ferdinand III. Thus, Empress Maria Anna was simultaneously his aunt and grandmother, while Margarita of Austria was both his grandmother and great-grandmother. One notable forebear of Charles', Joanna of Castile (also fondly known as 'Joanna the Mad') alone comprised two of his 16 great-great-great grandmothers, six of his 32 great-great-great-great grandmothers, and six of his 64 great-great-great-great-great grandmothers.

Nicknamed 'El Hechizado' ('the hexed') because of his deformities, Charles was saddled with an extreme version of the jutting, tapering Habsburg chin, as immortalised mercilessly in portraits by Titian and Velazquez, but it simply represented the business end of an oversized head, whose mouth struggled to contain an oversized tongue that left him with a propensity to jabber and slobber and an inability to chew. He also suffered from intestinal upsets, convulsions, premature ejaculation (according to his first wife) and impotence (according to his second). 'He was unable to speak until the age of four, and couldn't walk until the age of eight. He was short and weak, and very lean,' says Gonzalo Alvarez, who led the University of Santiago de Compostela study. 'He looked like an old person when he was 30 years old, bald and senile and suffering edemas on his feet, legs, abdomen and face. During the last years of his life he could barely stand up and suffered from hallucinations and seizures.' He finally succumbed at the age of 38, hairless and heir-less, taking the Habsburg line down with him, Hindenburg-style, as the French Bourbons moved in to fill the resulting power vacuum.

Other European royal houses have fared better - the occasional debilitating bout of haemophilia aside - despite the fact that, owing to the ever-shrinking pool of potential intra- blue-blood consorts, practically all of today's palace-dwellers can claim a direct descent from either Britain's Queen Victoria or King Christian IX of Denmark. The marriage of Elizabeth II of the United Kingdom and Prince Philip, Duke of Edinburgh (born a prince of Greece and Denmark), serves to illustrate a continent-wide family tree that more closely resembles a Byzantine bramble patch. To wit: Prince Philip is the son of Prince Andrew of Greece and Denmark and Princess Alice of Battenberg, whose mother, Princess Victoria of Hesse and by Rhine, and paternal grandfather, Prince Alexander of Hesse and by Rhine were both members of the same paternal family. Got that? So then, Princess Alice's paternal uncle, Prince of Battenberg, married Princess Beatrice (a daughter of Elizabeth II's great-great-grandmother, aka Queen Victoria). Their daughter, Victoria Eugenie of Battenberg, married King Alfonso XIII of Spain, and her grandson, the present king Juan Carlos, married Princess Sophia of Greece and Denmark, whose father was a cousin of Prince Philip, Duke of Edinburgh. Likewise, Queen Elizabeth's great-great-grandfather, King Christian IX of Denmark, was also Prince Philip's great-grandfather.

'Biologists have welcomed the Windsors' latter-day embrace of comparative commoners, and the sacrifice of 'royal mystique' in favour of genetic diversity.'

The queen and the duke are thus third cousins, through Victoria and Albert, and fourth cousins once removed, through King George II and Queen Charlotte, and, for good measure, are related several more times through Princess Sophia, Electress of Hanover. To many British people, and not just nascent republicans, this all helps to explain, say, Prince Charles' propensity for talking to trees, or the occasional unfortunate Hidden Cousin that comes to light (most recently, two female cousins of the queen, Nerissa and Katherine Bowes- Lyon, daughters of the Queen Mother's older brother, born with learning difficulties and exiled to an asylum in the British countryside in the 1940s). No wonder biologists have welcomed the Windsors' latter-day embrace of comparative commoners, in the form of Diana Spencer and Kate Middleton (although it's all relative - she and Prince William are actually 12th cousins, once removed), and the sacrifice of 'royal mystique' in favour of genetic diversity. 'It's a good thing,' says Francisco Ceballos of the University of Santiago de Compostela, who's also involved in the Habsburg study, adding that it'll guard against them following in the teetering pituitary-hormone-deficient and distal-renal-tubular-acidosistic footsteps of Charles II and his unfortunate ilk.

Recent commoner outreach programmes notwithstanding, it turns out that it was the politest members of society who were the most zealous inbreeders all along. But surely, in an age when mobility of all kinds, from social on upwards, is regarded less as a privilege than a birthright, the idea of anyone picking a prospective partner from the ranks of their adjacent-to- immediate family is a quaint but ill-advised notion that should be confined to the dustbin of history, along with witch-burning and frontal lobotomies? Well, yes, if we're talking literally; you'd find few out-and-proud intra-familial newlyweds these days taking the floor to the Elvis classic 'Kissin' Cousins', and singing heartily along to its somewhat disconcerting refrain: 'I've got a gal, she's as cute as can be/She's a distant cousin but she's not too distant with me... We're all cousins, that's what I believe/Because we're children of Adam and Eve.' But scientists suggest that the Habsburgs et al were just putting into admittedly over-literal practice a trait that may be hard- wired within us all, citing studies that show that, when it comes to choosing partners, people tend to opt for those who are strikingly similar to themselves - a phenomenon called assortative mating. The similarities range from social to psychological and physical, even down to earlobe length. Cousins, says Sir Patrick Bateson, a professor of ethology at Cambridge University, are a perfect e-fit for this human preference for 'slight novelty'. Whether prince or prole, it may be that the urge to keep those family ties super-taut may be more powerful and deep-rooted than we'd like to think, though the folkloric checks and balances are pretty compelling; the prospect of strollers full of hydra-headed bogey-spawn may be more of a 'slight novelty' than most societies, polite or otherwise, would be prepared to countenance.