More Kissing Cousins Are Marrying, New Studies Find : Lifestyles: Scientists say weddings between close relatives are rising in the U.S. Genetic cautions are real but less serious than generally thought.

BOSTON — Marriages between cousins and other close relatives are becoming more common in the United States and other Western countries as a result of immigration from countries where such marriages are accepted practice, scientists said here Sunday.

Several new studies suggest that the adverse genetic consequences of such inbreeding are real but are much less serious and widespread than is generally believed, researchers reported at a meeting of the American Assn. for the Advancement of Science. “There are ill effects, but they’ve probably been exaggerated in the past,” said geneticist James V. Neel of the University of Michigan.

Such problems, he and other researchers said, are typically outweighed by the cultural and socioeconomic benefits of such unions. “But we are interested because of the health consequences and the need for counseling,” Neel added.

These marriages, called consanguineous, are now prohibited in 30 states--although not in California--and carry heavy criminal penalties in nine. But the small increases in mortality resulting from them “don’t warrant such heavy-handed legislation,” said geneticist Alan Holland Bittles of the University of London.

“You can make a very good argument that inbreeding is not a bad thing,” Neel said. “The problems are more sociological than biological.”

Inbreeding is more common than many people suspect. An estimated 20% of marriages worldwide are between individuals who are first cousins or more closely related, with the incidence rising as high as 50% in countries such as Pakistan, Bittles said. In India, uncle-niece unions account for 20% of all marriages. And the number of consanguineous marriages is on the rise rather than declining, he added.

Such marriages play an integral part in the conservation of cultural values and property, he noted. “The question is not why there is so much inbreeding but why there isn’t more,” he said. “My guess is that there aren’t enough cousins to go around.”

Inbreeding is not biologically hazardous in and of itself, but rather for the fact that it brings out the effect of deleterious genes that might have otherwise gone unnoticed. Of the estimated 100,000 genes in each human, scientists now believe that as many as 60 are defective. Most such defects are “recessive”--they don’t produce any ill effects because the individual also inherited a good gene from the second parent.

But when two people with the same recessive gene mate, one of every four children will get two copies of the bad gene and develop the disorder. That disorder is often lethal early in life, but it can also cause such disabilities as congenital blindness, congenital deafness and lowered IQ, which do not threaten life.

Bittles has been studying the effects of consanguineous marriages in developing countries. In Pakistan, for example, he found that 16% of children from unrelated marriages died by age 10, while 21% of those from consanguineous marriages died by the same age.

“Another way to look at it is that 25% of the deaths among young children were the result of consanguineous marriages,” he said. He has observed similar results in other populations.

Geneticist Lynn B. Jorde of the University of Utah has taken advantage of the abundant genealogical records among that state’s Mormon population to study the problem. Among 405,595 members of the faith studied, 5,714 were the products of consanguineous marriages.

He found that among unrelated parents, 13% of the children died by age 16, compared to 22% of those resulting from consanguineous marriages. He noted that the death rates were high because the records extend back more than 100 years. Current death rates are much lower, he said, but the relative proportions remain the same.

Neel studied Japanese populations in Hiroshima and Nagasaki after World War II, where the rate of such marriages was 6% to 8%. The U.S. government was interested in the normal rate of genetic defects among those populations so that researchers could determine the number arising from exposure to radiation from the bombs used to end the war.

Neel found that 9% of the children from unrelated marriages died by age 10, compared to 10.5% from consanguineous marriages, a difference lower than he expected. “I’m still puzzled and confused,” he said. “Perhaps mutations are not as deleterious as we thought, or genetic material is more forgiving.”

All three agreed that inbreeding is not necessarily bad. “We’ve all had a certain amount of inbreeding in our pasts because we came from small population pools,” Bittles said. And the deaths of children with severe genetic defects, tragic as they may be, lead to the removal of bad genes from the gene pool.

“Perhaps it’s just Nature cleaning up the genome,” Neel said.