Contrary to widely held beliefs and longstanding taboos in America,
first cousins can have children together without a great risk of
birth defects or genetic disease, scientists are reporting today.
They say there is no biological reason
to discourage cousins from marrying.
First cousins are somewhat more likely than unrelated parents to
have a child with a serious birth defect, mental retardation or
genetic disease, but their increased risk is nowhere near as large
as most people think, the scientists said.
In the general population, the risk that a child will be born with a
serious problem like spina bifida or cystic fibrosis is 3 percent to
4 percent; to that background risk, first cousins must add another
1.7 to 2.8 percentage points, the report said.
Although the increase represents a near doubling of the risk, the
result is still not considered large enough to discourage cousins
from having children, said Dr. Arno Motulsky, a professor emeritus
of medicine and genome sciences at the University of Washington,
and the senior author of the report.
"In terms of general risks in life it's not very high,"
Dr. Motulsky said. Even at its worst, 7 percent, he said,
"93 percent of the time, nothing is going to happen."
The report is in today's issue of The Journal of Genetic Counseling.
"As genetic advisers," Dr. Motulsky said, "we give people all the
various possibilities and risks and leave it up to them to make a
decision. Some might decide a doubling of the risk is not
something they want to face.
He and his colleagues said no one questioned the right of people
with genetic disorders to have children, even though some have far
higher levels of risk than first cousins. For example, people with
Huntington's disease, a severe neurological disorder that comes on
in adulthood, have a 50 percent chance of passing the
disease to their children.
The researchers, a panel convened by the National Society of Genetic
Counselors, based their conclusions on a review of six major studies
conducted from 1965 to August 2000, involving many thousands of births.
Dr. Motulsky said medical geneticists had known for a long time that
there was little or no harm in cousins marrying and having children.
"Somehow, this hasn't become general knowledge,"
even among doctors, he said.
Twenty-four states have laws forbidding first cousins from marrying,
and seven states have limits like requiring genetic counseling.
But no countries in Europe have such prohibitions,
and in parts of the Middle East, Africa and Asia,
marriages between cousins are considered preferable.
"In some parts of the world," the report says, "20 to 60 percent
of all marriages are between close biological relatives."
Dr. Motulsky said many immigrants from cultures where cousin
marriages are common expect to continue the tradition in the
United States, and doctors and genetic counselors
should respect their wishes.
Laws against cousin marriage should be abolished, he said. Even
though longstanding ones reflect a view that such marriages are
"really bad," he said, "the data show it isn't that bad."
Dr. Motulsky said researchers did not know why marriage between
cousins was viewed with such distaste in the United States. He said
some of the revulsion might have stemmed from the eugenics movement,
which intended to improve the human race by deciding who should be
allowed to breed. The movement flourished in this country
early in the 20th century.
It is not known how many cousins marry or live together. Estimates
of marriages between related people, which include first cousins and
more distant ones, range from less than 0.1 percent of the general
population to 1.5 percent. In the past, small studies have found
much higher rates in some areas. A survey in 1942 found 18.7 percent
in a small town in Kentucky and a 1980 study found
33 percent in a Mennonite community in Kansas.
The report made a point of saying that the term "incest" should
not be applied to cousins but only to sexual relations between
siblings or between parents and children. Babies who result from
those unions are thought to be at significantly higher risk of
genetic problems, the report said, but there is
not enough data to be sure.
The new report says that genetic counselors should advise cousins
who want to have children together in much the same way they
advise everybody else and that no extra genetic tests are
required before conception.
The guidelines urge counselors to take a thorough family history
and, as they do for all clients, look for any diseases that might
run in the family or in the clients' ethnic groups and order tests
accordingly. During pregnancy, the woman should have the standard
blood tests used to screen for certain neurological problems and
other disorders and an ultrasound examination.
Their children should be tested as newborns for deafness and certain
rare metabolic diseases -- tests already given to all newborns in
some parts of the country. These are among the conditions that may
be slightly more likely to occur in children whose parents are
cousins. Some of the metabolic problems are treatable, and children
with hearing losses do better if they get help early in life.
Dr. Motulsky said that the panel of experts began working on the
cousin question about two years ago after a survey of counselors
found a lot of variability -- and misinformation -- in the advice
given to people who wanted to know whether cousins could
safely have children together.
The president-elect of the National Society of Genetic Counselors,
Robin L. Bennett, who is a co-author of the report and a genetic
counselor at the University of Washington, said: "Just this week I
saw a 23-year-old woman whose parents were cousins, and she was told
to have a tubal ligation, which she did at the age 21, because of
the risk to her children. And there's no risk to her children.
People are getting this information from small-town doctors who may
not know the risk, don't have access to this information
and just assume it's a big risk."
The young woman hopes to have the operation reversed, Ms. Bennett
The article in the geneticists' journal includes a personal account
from a woman who said she had lived with her cousin for six years,
"and we are madly in love." When she became pregnant, she said,
her gynecologist warned that the child would be sickly and urged her
to have an abortion. A relative predicted that the baby would be
retarded. She had the abortion, she said, and called it
"the worst mistake of my life."
When she learned later that the increased risk of birth defects was
actually quite small, she said, "I cried and cried."
The small increase in risk is thought to occur because related
people may be carrying some of the same disease-causing genes,
inherited from common ancestors. The problems arise from recessive
genes, which have no effect on people who carry single copies, but
can cause disease in a person who inherits two copies of the gene,
one from each parent. When two carriers of a recessive gene have a
child, the child has a one-in-four chance of inheriting two copies
of that gene. When that happens, disease can result. Cystic fibrosis
and the fatal Tay-Sachs disease, for example, are caused by
recessive genes. Unrelated people share fewer genes and so their
risk of illness caused by recessive genes is a bit lower.
Keith T., 30, said he married his cousin seven years ago and in
1998, frustrated by the lack of information for cousins who wanted
to marry, he started a Web site,
It is full of postings from people who say they have
married their cousins or want to do so.
The site highlights famous people who married their first cousins,
including Charles Darwin, who, with Emma Wedgwood, had 10 children,
all healthy, some brilliant. Mr. T. asked that his full name not be
used because he said he did business in a small town
and feared that he would lose customers if they found out his wife
was also his cousin.
"If someone told me when I was young that I'd marry my cousin I
would have said they were crazy," he said. "I thought the idea of
marrying your cousin was kind of icky."
Mr. T. said he was relieved to learn years ago that cousins' risks
of birth defects, while higher than those of unrelated people,
were still relatively low, and that he and his wife
hoped to have children.
~ Copyright 2002 The New York Times Company