Is
marrying close relations harmful? Charles Darwin, anxious about his own
family inbreeding, applied his scientific rigour to find out the truth.
Adam Kuper reveals all
Charles
Darwin’s mother, unwell throughout his childhood, died from an
agonising stomach ailment, probably peritonitis, at the age of 52.
Charles was then eight years old. As an adult he was obsessively
concerned with his own ill-health, particularly the recurrent stomach
complaints that recalled his mother’s fatal illness.
Was
his affliction hereditary? Darwin’s mother was a Wedgwood. and the
Wedgwoods were notorious for their ill-health. Whenever one of his
children fell ill, Charles was inclined to see the same symptoms in
himself, and to worry that it exposed a family propensity.
Or
were the frequent illnesses of his children perhaps the consequence of
inbreeding? He had married his cousin, Emma Wedgwood, his mother’s
brother’s daughter. Is that why his children suffered, and his beloved
daughter Anne had died in childhood? The poor health of the whole
Wedgwood clan might be explained by a long-standing preference for
marrying cousins.
These were not simply personal worries. The
risks of cousin marriage became a subject of scientific discussion in
the 1860s. And in Darwin’s case, his scientific project and his private
concerns fed on each other.
Between 1868 and 1877 Darwin
published three monographs on cross-fertilisation in animals and plants.
In the first of these books,
The Variation of Animals and Plants under Domestication,
he proposed that “the existence of a great law of nature is almost
proved; namely, that the crossing of animals and plants which are not
closely related to each other is highly beneficial or even necessary,
and that interbreeding [i.e., inbreeding] prolonged during many
generations is highly injurious.”
Was this also true of human
beings? Darwin thought this probable. He was accordingly fascinated by a
study of cousin marriage in Scotland published in 1865 by Arthur
Mitchell, Deputy Commissioner in Lunacy for Scotland.
It was
widely believed that marriage between close relatives was rampant in
remote Scottish regions, particularly the Highlands and Islands.
However, Mitchell noted that popular opinion in Scotland condemned
“blood-alliances” as “productive of evil”. And indeed national
statistics showed that nearly 14 per cent of “idiots” in Scotland were
children of kin. In 44 per cent of families with more than one mentally
handicapped child the parents were blood relatives. Six per cent of the
parents of deaf mutes were close relatives.
Yet Mitchell was not
convinced that this was the whole story. Fewer than two per cent of
marriages in Scotland were between first or second cousins. The rate was
indeed higher in some isolated regions, but the evidence for bad
effects was inconclusive. In one small town on the north-east coast of
Scotland, nine per cent of marriages were with first cousins and 13 per
cent with second cousins. Mitchell acknowledged that the children of
these cousin marriages were often unprepossessing, but then many fishing
families in the region were “below par in intellect”. A more telling
case was Berneray-Lewis (now Great Bernera, off the Isle of Lewis). Here
11 per cent of marriages were with first and second cousins, yet
Mitchell remarked that “instead of finding the island peopled with
idiots, madmen, cripples, and mutes, not one such person is said to
exist in it”.
Perhaps environmental factors – “occupation, social
habits, etc.” – influenced the outcome. One “shrewd old woman” remarked
to him: “But I’ll tell ye what, Doctor, bairns that’s hungert i’ their
youth aye gang wrang. That’s far waur nor sib marriages.” Mitchell
concluded that close-kin marriage tended to reinforce “evil influences”.
In
1870 Darwin’s ally John Lubbock proposed an official investigation of
the effects of cousin marriage, but could not persuade parliament.
Darwin then decided to commission a private study of the topic. He
entrusted it to his eldest son, George, who was an accomplished
mathematician.
Darwin laid out the research design himself.
George was first to establish the incidence of close-kin marriage in the
general population. He was then to enquire how many patients in insane
asylums were the children of close relatives. If it turned out that
marriages between close relatives produced a disproportionate number of
“diseased” children, he instructed George, this would “settle the
question as to the injuriousness of such marriages”.
The first
step was to find out how common it was in England for first cousins to
marry. Apparently nobody knew the answer. George Darwin was given
estimates that ranged from 10 per cent to one in a thousand. “Every
observer”, he concluded, “is biased by the frequency or rarity of such
marriages amongst his immediate surroundings.” He would have to discover
the facts for himself. Expert in the new statistical techniques that
were being developed by his own cousin Francis Galton and by William
Farr, George decided to attempt a scientific survey. It was to be one of
the very first statistical studies of a social problem.
Using
marriage records, press announcements of marriages, and questionnaires,
George Darwin discovered that about 4.5 per cent of marriages in the
aristocracy were with first cousins; 3.5 per cent in the landed gentry
and the upper-middle classes; about 2.25 per cent in the rural
population; and among all classes in London, about 1.15 per cent.
The
next step was to gather statistics from mental asylums. Charles Darwin
wrote on George’s behalf to the heads of the leading institutions.
Several provided detailed responses. These indicated that only three to
four per cent of patients were the offspring of marriages between first
cousins. This was not out of line with the incidence of first-cousin
marriage in the population at large. George concluded that “as far as
insanity and idiocy go, no evil has been shown to accrue from
consanguineous marriages.”
Other studies suggested that the
offspring of cousin marriages were more likely to suffer from blindness,
deafness, or infertility. George accepted that these conditions were
highly hereditary, but saw no convincing evidence that they were caused
by cousin marriage. In fact, first-cousin marriages were, if anything,
more fertile than others. He suggested that a man was more likely to
marry a cousin if he had many to choose from. First-cousin marriage
would therefore be more common among people who came from large – and so
presumably fertile – families.
Only one small piece of evidence
gave George pause. Among men who had rowed for Oxford or Cambridge, men
who were obviously the fittest of the fit, sons of first-cousin parents
appeared slightly less frequently than might have been expected (2.4 per
cent as opposed to 3-3.5 per cent among their peers).
To be
sure, George Darwin was well aware that his conclusions flew in the face
of a common and ancient prejudice. He conceded that marriages between
cousins might be quite all right for the rich but bad for the poor.
After all, “my father, Mr Charles Darwin, has found that in-bred plants,
when allowed enough space and good soil, frequently show little or no
deterioration, whilst when placed in competition with another plant,
they frequently perish or are much stunted.” In short, cousin marriages
caused no harm in the best families, but the inhabitants of slum
tenements should probably avoid them.
Charles Darwin endorsed
these conclusions. In later editions of Variation he modified his
original rule, weakening the claim: “it is a great law of nature, that
all organic beings profit from an occasional cross with individuals not
closely related to them in blood.” (Emphasis added.) On the other hand,
the experience of animal breeders indicated that the advantage of
inbreeding “as far as the retention of character is concerned, is
indisputable, and often outweighs the evil of a slight loss of
constitutional vigour”.
The
densely intermarried Wedgwoods
liked to joke that any hint of laziness or illness was an infallible
sign of familial degeneracy. However, the Darwinian establishment was
now convinced that the risks of cousin marriage were slight, at least
within prosperous families. Francis Galton wrote enthusiastically to
George Darwin that he had “exploded most effectually a popular scare”.
He added that his cousin could make a fortune from his discovery.
“Thus:
there are, say, 200,000 annual marriages in the kingdom, of which 2,000
and more are between first cousins. You have only to print in
proportion, and in various appropriate scales of cheapness or luxury:
WORDS of Scientific COMFORT and ENCOURAGEMENT to COUSINS who are LOVERS
then each lover and each of the two sets of parents would be sure to buy
a copy; i.e. an annual sale of 8,000 copies!! (Cousins who fall in love
and don’t marry would also buy copies, as well as those who think that
they might fall in love.)”
Galton’s protégé Karl Pearson, a
stalwart of the eugenics movement, made a follow-up study in 1908. He
was less systematic than George Darwin, relying on information
volunteered by readers of the British Medical Journal. These select
respondents reported a very high incidence of first-cousin marriages in
their own families. A smaller proportion of marriages were with more
distant cousins, but Pearson remarked that second and third cousins in
these families were often related in more than one line. He lumped them
all together and concluded that “consanguineous marriages in the
professional classes probably occur in less than eight per cent and more
than five per cent of cases.” Yet only 1.3 per cent of patients in the
Great Ormond Street Hospital for Children were the children of cousins.
Pearson concluded that “the diseases of children are not largely due to
any consanguinity between their parents.”
Endorsed by the
Darwinian establishment, George Darwin’s conclusions reassured many
people whose family trees featured marriages between cousins. Englishmen
could also rest more easy when they considered that Queen Victoria was
married to a first cousin, and that several of her descendants had also
married cousins. And Darwin’s conclusions seemed only common sense to
landowners in the House of Lords, who knew that the inbreeding of good
stock was sound policy.
American scientists disagreed, however.
The Bemiss Report to the American Medical Association in 1858 claimed
that marriages between cousins were responsible for a number of birth
defects. Despite its slapdash methodology, the Bemiss Report got wide
publicity. Politicians and journalists began to demand a ban on cousin
marriage. Judges and clergymen weighed in with solemn warnings.
Kansas
banned the marriage of first cousins in 1861. Ten of the states that
joined the union in the second half of the 19th century passed similar
legislation (although not California or Texas). Several of the older
states introduced a ban on first-cousin marriage, beginning with New
Hampshire in 1869. Others, notably Massachusetts, New York and
Pennsylvania, still allowed cousins to marry, but everywhere cousin
marriage became less common. (First-cousin marriage has since been
banned in Kentucky (1946), Maine (1985) and Texas (2005).)
he
reaction against cousin marriage gained ground in Britain after the
First World War, and by the 1920s British eugenicists were routinely
condemning cousin marriage. Leonard Darwin, another son of Charles
Darwin, followed his cousin Francis Galton as President of the Eugenics
Education Society and joined the chorus of disapproval, despite the fact
that he was himself the son of cousins and had married a first cousin
once removed. In the 1930s the liberal geneticists JBS Haldane and
Lionel Penrose denounced eugenic racial theories. Nevertheless they
agreed that cousin marriages should be discouraged because of their link
to recessive disorders, notably congenital deaf-mutism and certain
mental defects.
In the English upper and upper-middle classes the
prevalence of first-cousin marriage had remained steady at between 4
and 5 per cent for much of the 19th century – that is, one marriage in
every twenty to twenty-five. Then quite suddenly, after the First World
War, cousin marriage became very unusual. By the 1930s, only one
marriage in 6,000 was with a first cousin. A study of a middle-class
London population conducted in the 1960s found that just one marriage in
25,000 was between first cousins.
There are many reasons for
this shift, not least the decimation of young men in the trenches of
Flanders. The changes in scientific opinion were probably not decisive.
After all, Darwin had played down the risks to offspring. However, as
the practice changed so did the beliefs. Today there is a common feeling
that a liaison between cousins is incestuous, and that if it is not
forbidden it should be, if only because it carries unacceptable risks of
genetic damage to offspring. And yet the consensus amongst experts
today is much closer to the conclusions of George Darwin than it is to
the popular view.
Any mating is risky to some degree, one danger
being that offspring may be born with a defect if parents have a
deleterious recessive gene in common. The chances of birth defects and
of infant mortality are roughly doubled for the children of first
cousins, but in normal circumstances that means that only some
additional two per cent of children may be affected. Reviewing a number
of modern studies, the geneticists AH Bittles and E Makov concluded that
“the risks to the offspring of inbred unions generally are within the
limits of acceptability. For first cousin progeny, it also must be
admitted that they appear to be in remarkably close agreement with the
levels calculated by [George] Darwin in 1875”. In the USA, the National
Society of Genetic Counselors recently convened a panel of experts to
review the risks of first-cousin marriage. They reported that the small
background risk of congenital defects is raised by some 1.7 to 2 per
cent in the case of children of first cousins. There is also an
additional 4.4 per cent chance of pre-reproductive mortality.
The
risks are significantly higher if cousin marriages are repeated over
several generations. This may be a factor in the high prevalence of
birth defects in the British Pakistani population, amongst whom cousin
marriages are common.
In Pakistan, and in the Pakistani diaspora,
a preference is widely expressed for marriage within the extended
family or biradari, and in fact marriages between close relatives are
common in most regions of Pakistan. Perhaps unexpectedly, the rate of
cousin marriage is even higher among Pakistani immigrants to Britain
than it is in rural Pakistan. And the present generation of British-born
Pakistanis has the highest rate of all. Around a third of the marriages
of the immigrant generation were with first cousins, but well over half
the marriages of the British-born generation are with first cousins.
A
preference for cousin marriage is often put down to religious dogma,
but the real reasons are in every sense closer to home. The remarkable
preference of British Pakistanis for cousin marriage is largely a
consequence of immigration, and more specifically of British immigration
regulations. It is very difficult for people to enter Britain unless
they are married to people already here. In most cousin marriages, one
partner is brought to Britain from Pakistan. Alison Shaw found that 90
per cent of the first-cousin marriages in a sample of British Pakistanis
in Oxford involved one spouse who came directly from Pakistan. There
are often debts to family members back home, who helped to finance the
migration. Above all, there is family loyalty. Roger Ballard points out
that if a British-based family refuses a marriage offer from relatives
in Pakistan, “they are likely to be charged with having become so
anglicised that they have forgotten their most fundamental duties
towards their kin”.
The odds of genetic misfortune rise sharply
if cousin marriages are repeated over several generations, but it is not
easy to separate the effects of cousin marriage, heredity and
environment. One study of the genetic consequences of cousin marriage in
families of Pakistani origin in Norway found that the risks were rather
low. Other studies are less reassuring. The authors of a recent
prospective survey of Pakistani families in Birmingham suggests that if
they “ceased to marry relations, their childhood mortality and morbidity
would decrease by 60 per cent”.
The tabloid press – and populist
politicians – claim that the preference for cousin marriage overloads
the health services and prevents integration, and put the blame on
superstitious beliefs and patriarchal attitudes. And yet the family life
of British Pakistanis is close to the Victorian ideal. Indeed, the
great bourgeois clans of Victorian England – like the Darwin-Wedgwoods –
had a great deal in common with a modern Pakistani biradari. The
19th-century English bourgeoisie would have been sympathetic to the
motives that incline these new Britons to marry their cousins.
Source: New Humanist
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