European aristocrats, like those of ancient Egypt, have long married their kin. In a world in which nobody mates with a relative, and with the births of parents and children separated by twenty to thirty years, each reader of these pages could have had, at the time of Charles Darwin’s birth seven generations ago in 1809, a hundred and twenty-eight different ancestors - two multiplied by itself seven times. For almost everyone that figure is too high, for some marriage among those who have no idea that their spouse is a distant relative is inevitable. Even so, social pressure for matrimony within the household can do a lot to reduce that number. Alfonso, the Infante of Spain, who died in the 1960s, had just twelve - rather than more than a hundred - ancestors seven generations back, King Alfonso XII, a contemporary of Darwin, had sixteen, while plenty of others in that noble line had between fifteen and twenty great-great-great-great-grandparents - far fewer than expected in a sexually open society. The Spaniards are still keen on the pastime, with levels of cousin marriage well above the European average, and their nation has the most inbred villages on the continent, Asturias in the north being the most inward-looking place of all.
The habit also thrives elsewhere. Many of India’s thousands of communities insist on alliances within their own group. The Dravidian Hindus of the south have encouraged sex between cousins, or between uncles and nieces, for two thousand years, and continue to do so, with one marriage in ten between a man and his brother’s daughter. In large parts of Africa and the Middle East, a fifth of all alliances - and in some places even more - is between close relatives. The practice is also common in the world of Islam. The Prophet discouraged the idea, but he did marry off his daughter Fatima to her cousin Ali, which in the eyes of the faithful legitimises the habit. The tradition is not, contrary to popular belief, central to that religion but has become part of its culture. In Bombay, one Hindu marriage in fifteen is between close relatives, but for Muslims the figure is three times as high. Among the nomadic Qashqai people of Iran, the incidence of cousin marriage is three in four.
The picture in the West is quite different. Most of us rarely meet our relatives, let alone sleep with them. In most places less than one marriage in a hundred is between cousins. The English have long been among the least inbred nations in Europe. George Darwin had imagined that farm labourers ‘would hold together very closely’ but he was surprised to find how few weddings among kin there were in the rural districts of his time. England has never had a peasant class that sits on its own land for centuries and falls for the girl next door for lack of a better candidate. John Bull was a tenant. His master often forced him off and provided, as an unexpected bonus, a wider choice of sexual partners. In modern Britain, husbands and wives are, on the average, sixth cousins. Most share no more than a great-great-great-great-great-grandparent in common - someone who lived before Charles Darwin was born - and most have no idea at all that they are related. Only on islands, real or metaphorical, do we go in for the habit. Many Northern Irish travellers and a fifth of the people of parts of the Hebrides choose a first or second cousin as a mate. On Darwin’s island today, faith is a better barrier to sex than are miles. In Bradford, the Pakistani community is among the most inbred in the world, with its British-born children having an even higher chance of marrying a cousin (often one still in Pakistan) than did their parents.
Social pressures can also reduce the extent of inbreeding. In many places the prospect of prison for sex with a cousin has, no doubt, often put paid to romance. In hunter-gatherer days, men and women roamed the landscape to about the same extent. With agriculture, life changed. The geography of men and women as seen in the patterns of Y-chromosome and mitochondrial genes shows that the males tended to stay at home while their mates came from elsewhere. The son inherits the capital and does not wish to move or share with a neighbour. He prefers a wife from far away who is, as an incidental, unlikely to share his genes.
Close inbreeding can - as Darwin had found in the greenhouse - impose a real burden. All human populations contain damaged genes, manifest only when inherited in double copy. The children of cousins may pay the price for that legacy. As Bagehot wrote: ‘It has been said, not truly, but with an approximation to truth, that in 1802 every hereditary monarch was insane’ and inbreeding was at least in part to blame. Every reader of these pages carries at least one gene in single dose that would kill them if present in double copy. Most inborn faults are hidden by normal versions of the same gene; for example, one British child in two thousand five hundred is born with two doses of the damaged gene that leads to cystic fibrosis, but one Briton in twenty-five has a single copy. If relatives mate and if their common ancestor bore a faulty piece of DNA, the chance that each partner will inherit that fault by virtue of shared descent goes up. Their children are then at increased risk of receiving two damaged versions.
The malign effects of sex within a closed pool were noticed long ago. Umar Ibn Al-Khattab, the second Caliph and direct follower of Muhammad, advised members of a certain tribe to marry out because, he believed, they had become weak and unhealthy through their habit of sex with kin. Its power became manifest in 1908 when Sir Archibald Garrod identified an inborn illness called alkaptonuria as a condition in which two copies of a faulty piece of DNA were needed to show their effects. An enzyme that breaks down certain food substances is damaged. Symptoms include dark ear wax, smelly urine and, later in life, heart, skeletal and other problems. The disease is rare, with just one case in every twenty thousand births, but Garrod found that more than half of the parents of his patients were cousins. The same is true for other such conditions. In France, with one marriage in five hundred between first cousins, the incidence of cystic fibrosis is over-represented by seven times in the children of such matings.
In an unfortunate coincidence, certain places with a lot of sex among kin - North Africa, the Middle East and the Indian subcontinent - also have a high incidence of inherited blood diseases that protect against malaria. Sickle-cell is carried in single dose by almost half the members of some African populations and related errors are almost as common in other places. Each is dangerous when inherited in double copy.
In Saudi Arabia, where in some villages eight out of ten people marry their cousins, such diseases are common. A fifth of all admissions to children’s wards are due to hereditary disorders. Many families are unaware of the dangers and the devotion to cousin marriage remains. Doctors now advise those at risk to screen pregnancies in the hope that damaged foetuses might be picked up before they are born and the government insists that engaged couples are tested to see if both carry a blood disorder. No overt pressure is brought to bear, but the incidence of marriages within the family has dropped by a fifth.
Even in places without high levels of inborn disease children born to cousins die younger than usual. Such unions in Utah Mormons - not themselves an unusually inbred population - from the mid-nineteenth to the mid-twentieth century led to a notable increase in ill health. The effects became worse as the infants grew older, perhaps because death in old age has a stronger genetic component than do the accidents of infection or starvation that killed the pioneers’ babies. Heart disease is also more frequent among the children of cousins. A study of half a million pregnancies in the modern United States suggests that the death rate of the sons and daughters of cousins rises by about 5 per cent above average. The products of uncle-niece marriages, a pattern frequent in India and elsewhere, do even worse and while incest - sex between sibs, or parents and children - is rare the offspring pay a high price. A German brother and sister, adopted at birth and strangers until they met as adults, had four children, two of whom were severely affected. A study of thirty or so Canadian children born to such parents also suggests that almost half inherit some abnormality.
A more subtle, but more marked, effect of within-family sex has emerged in Iceland. Among a hundred and fifty thousand couples born between 1800 and 1965, partners who were close relatives had more, rather than fewer, children than average. Even so, the proportion of the children of first and second cousins who themselves reproduced (and hence the number of grandchildren born to the pair of relatives) was well below average, in part because many of those first-generation progeny died young. Charles Darwin and his cousin Emma may have been testimony to that effect, for seven of their ten sons and daughters expired before their time or lived on but stayed childless. Close mating may be more harmful to a family’s prospects than was once supposed.
Continued inbreeding leads to a decrease in variability within a lineage. The effects soon extend across the entire genome. The overall level of DNA variation is lower in people who emerge from a limited pool of ancestors and, as a result, the level of inherited diversity gives an insight into the extent of inbreeding. Many illnesses - diabetes, heart disease and more - are more frequent, and more severe, among those so revealed to have a history of sex with kin. In Bradford, some members of the Pakistani community are uniform in long stretches of their genome. They pay the price in terms of health and even their general liability to infection goes up. In Darwin’s day, childhood death came in the main from contagious disease. His beloved Annie died of tuberculosis (although the diagnosis was the obscure ‘bilious fever with typhoid character’) but her plight may, as he feared, in part have been due to her parents’ marital history.
Sex is, needless to say, more complicated in the bedroom than in the greenhouse and the simple fit of health with kinship does not always hold. In some places, relatives marry to keep wealth within the household, which means that there must be some wealth around in the first place. In parts of India and Pakistan, cousin marriages are commoner among the affluent than among the poorest, for the latter have no financial incentive to set up home with a relative. The effects of cash outweigh those of genes. As a result, the children of cousins are less, rather than more, liable to suffer ill health. As is true for the sheep on Soay, the environment also plays a part. In the poor and embittered Japan of the 1940s, cousin marriage had a large influence on infant welfare, but twenty years later the effect had almost disappeared.
For both plants and people, sex (when not with oneself) must involve another party. Almost always, he or she must choose or be chosen from a pool of potential mates. The process calls for hard decisions. Some are obvious. Whites tend to marry whites and blacks blacks, while the rich marry the rich and the tall the tall and, some say, men tend to find a wife who looks rather like their mother. Plenty of religions make it hard to find a partner outside the creed. Language, place of birth, education and more also affect the choice of candidate. All this means that for any man or woman the number of possible mates is far smaller than it might appear to be. That observation is familiar enough, but biology sharpens up human sexual decisions in ways both obvious and less so.
The fundamental question about sex is: why bother? The habit is expensive for sex reduces the number of potential mates. It imposes the simple rule that only some individuals - those of a different gender from oneself - are available for copulation. Self-fertile plants circumvent that demand, but sexual creatures (ourselves included) have their choices much restricted by it. Often the rules become even more stringent. Evolution generates laws that ensure that no longer can any male mate with any female (or vice versa). They ensure, as a result, that an individual of one sex will be accepted by no more than a fraction of those of the other. Sex becomes more sexual than before. The process is driven by the need to avoid inbreeding.
Darwin discusses such issues in his book
The Different Forms of Flowers on Plants of the Same Species,
published a year after the volume on self-fertilisation. It begins with a simple tale: the story of the children of the village of Downe, who made necklaces from cowslips. They could, they told him, use just a few of the plants, those with a long ‘pin’ that protruded from the flower, through which they could thread the plants together. Other flowers, instead of a pin, had no more than a short protrusion called a ‘thrum’ and were of no use as juvenile jewellery. The cowslip’s close relative, the primrose, was much the same.
Pins and thrums represent an additional mechanism of sex choice. Female ‘pin’ flowers were, Darwin found, much happier to accept pollen from male ‘thrums’ than they were from males of their own kind. The same applied in the opposite direction. A male needs a female, but the cowslip asks for more. The flower’s form is inherited - which means that the plants decide whether or not to accept another’s pollen advances on genetic grounds. Like mates only with unlike, so that a second sexual filter reduces the chance of an encounter between plants that bear similar genes. It is, as a result, a precaution against inbreeding. With pins and thrums, Nature has come up with a trick to reduce the proportion of individuals with whom genes can be shared. She has, in effect, invented more and more sexes.
Darwin’s greenhouse experiments on cowslips have grown into a science that shows how, in both primroses and people, partners are chosen in unexpected ways and that the choice may decrease the prospects of successful mating between those who have recent ancestors in common. Thirty or so plant groups have evolved systems rather like that of the cowslip and primrose. He found others in which the flowers came in not two but three forms, each of which would not accept a partner from within their own class.
Now we know many more examples of such physical barriers to sex. Some species produce flowers that are mirror images of each other and can cross only left to right and right to left. In a bizarre system found in tropical gingers, some individuals are male in the morning and female in the afternoon while others prefer the opposite pattern. As a result they are obliged to exchange genes with those whose sexual timetable is different from their own. Once again, the imperative is to avoid carnal relations with those like themselves - with kin.