Hi dnadna -
Your question is very interestingly worded. Are there studies which
show that interbreeding is harmful? Certainly the answer among
animals is yes. Or maybe, I should say yes, but!
THE GENETICS OF CLOSE RELATIVE BREEDING
Geneticists, dating back to Gregor Mendel in the 1800's have shown
that if they selectively breed for a certain gene, that gene will
increase in the population. Perhaps the least esoteric example coming
to mind is dog breeding. If a breeder keeps selecting smaller parents
to breed, eventually the breeder will have a population of tiny dogs
in his kennels. The breeder has done this by breeding close relatives
with the needed trait. Eventually a small (or whatever) dog is
Now imagine a human population with a harmful but hidden (recessive)
trait. While no one can manipulate the population to achieve high
frequencies of the trait, it will appear if carriers of the recessive
trait mate with each other. Many studies have shown that a harmful
hidden trait will show itself (by being present in a double recessive
dose) in families formed by cousin marriages.
In fact it is a generic genetic "rule" that consanguinous
marriages--between close relations --increase the chance of previously
hidden genes will come to the fore:
When Parents are Relatives: Consanguinuity
These hidden genes may be benign, and two cousins, say, will have
healthy babies. But if the previously hidden gene is harmful,
developmental defects will show up in the offspring.
Some examples are:
MULTIPLE SCLEROSIS: first cousin marriages in Sweden
HEARING IMPAIRMENT: first and second cousin marriages in Riyadh, Saudi
HEART DEFECTS: first, second, and third cousin marriages in a Bedouin
tribe in Israel
BRAIN DEFECTS: first cousin marriage in Afghanistan
SCALP AND LIMB DEFECTS: first cousin marriage in Australia
The prohibition of sibling and cousin marriages antedates even the
19th Century knowledge of genetics. In Europe the Catholic Church
forbade marriage not only between brothers, sisters, and cousins, but
also with a godparent or even son or daughter of a godparent.
Consanguinity in Canon Law
Today, in the United States, marriage laws are promulgated by the
States, and each differs with respect to marriage between relations.
In all, according to CousinCouples.com, 16 States permit cousin
marriages with no exceptions; an additional 7 permit them with some
exceptions (for example, a blood test for certain genetic diseases).
Under Australian law, marriage between people who are of "direct blood
line," that is, brothers/ sisters, brothers/ half sisters, sisters/
half brothers, grandparents/ grandchilden are forbidden.
Genetics Fact Sheet
In Canada, parents cannot marry their blood-related children nor can
blood- related siblings marry.
I hope this answer has been helpful to you. Do ask for a
clarification if needed.
All the best,
SEARCH TERMS USED IN GOOGLE SEARCHES: civil law consanguinity;
inbreeding siblings cousins; "animal breeding" negative traits;
"gregor mendel" experiments; "dog breeders" weaknesses; "cousin
SEARCH TERM USED IN PUBMED (NATIONAL LIBRARY OF MEDICINE) SEARCH:
consanguinity siblings cousins