"In some parts of the world,' the report says, '20 to 60 percent of all marriages are between close biological relatives."
Probably isolated areas, like small islands or remote settlements in the wilderness where there is not much to choose from.
I've lived a year in such a place and, while there weren't genetic illnesses that I was aware of, I wouldn't use that place as evidence of how inbreeding is innocuous. Let's leave it at that.