The disturbing thing that happens when you tell people they have different DNA

The first real interview I ever did was in 1999 in Dubrovnik, Croatia, the white-stone city perched on the Adriatic Sea that is now famous as King’s Landing in the TV series “Game of Thrones.” But less than 10 years before, that beautiful city had been the site of a siege, in a war that turned the former Yugoslavia against itself.

I had stepped into a Serbian Orthodox Church, cool and dark against the bright sun outside. The church’s caretaker, an older man with a massive white mustache, told me how the city had descended into chaos during the conflict, which pitted mostly Catholic Croats against mostly Orthodox Serbs and Muslim Bosniaks in the early 1990s. Former friends and neighbors spurned one another until there was no place in the community where the Orthodox caretaker and his Catholic wife were accepted, he said, with tears falling. The church was still damaged from the bombings.

One of the great mysteries of ethnic conflict is how it can suddenly and drastically cleave into groups people who formerly considered themselves one community. In...