"I read a study about ten years ago, about in breeding within the Japanese Christian community. Which, due the small number of members, was very interbred...."
I have to seriously question this. The number of Japanese Christians is high enough that I don't see how inbreeding would be a problem at all. (No more so, certainly, than for Jews or Mormons or Jehovah's Witnesses or Muslims in the United States.)
According to my former Baptist pastor, when he attended the World Baptist Convention in Tokyo in the 1960s "THOUSANDS of Japanese Baptists attended, as delegates, volunteers and spectators."
And that's just ONE Christian demomination out of many, and not even the largest.
During World War Two the Japanese government issued New Testaments to its Christian soldiers! In fact, one of the best-kept secrets of World War Two was the fact that a good number of the Japanese pilots shot down at Pearl Harbor on December 7, 1941, were carrying New Testaments! (I've never been able to find out the exact percentage.) The US Government didn't want word to get out to the American people that there were CHRISTIANS among the attackers.
PS Felix, could your author have been writing about a specific Christian denomination, small in size, which doesn't believe in intermarriage iwith other Christians? There is, for example, a sect known as "secret' or "hiding" Christians which was formed as a "catacomb"-style church during the few anti-Christian persecutions of centuries ago and remains so even though the group has been perfectly legal for hundreds of years.