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As incestuous marriages become more common in the United States, scientists are unprepared to offer counseling about the likelihood of genetic disorders, a University of Michigan researcher said.

"I'd been brought up to believe that inbreeding was pretty nasty," said Alan Bittles, a geneticist who has spent 10 years researching interfamily marriages in Pakistan and India. "We never consider that in other parts of the world it's perfectly normal to marry a cousin."Bittles said he has determined that 500 million people worldwide are married to relatives, typically uncles and nieces or first or second cousins.

"All those have got to be rock-bottom low estimates, because we're missing information from so many parts of the world," he said.

And as people move to the United States and other Western nations, traditions follow.

Although there are fewer genetic disorders among children born to married family members than Westerners believe, scientists have made few studies of those problems and have little information about them, he said.

Inherited disorders include some blood diseases, protein deficiencies, spina bifida, cleft palates and hip and heart deformities.

If parents from the same family marry and seek genetic counseling, doctors can give only occurrence rates derived from general population studies, Bittles said.

"We may be giving wrong advice," he said. "We would be hard-pressed in this community to tell them what their risks are."

Bittles, a Fulbright Fellow in the University of Michigan Population Studies Center, is a geneticist from Kings College in London. He is one of only a few researchers studying intermarriage but expects the field to grow.

"Once people recognize that this is a subject that needs investigation, that it has tremendous importance on a community, then they'll start studying it," he said.