I actually don't recall where I read that. It was one of the following 2 books:

https://www.amazon.com/Sapiens-Humankind-Yuval-Noah-Harari/d...

https://www.amazon.com/Tribe-Homecoming-Belonging-Sebastian-...

I wouldn't be surprised if the book derived that number from the link you gave either.

A recent book that explores the loss of community when combat veterans come home, and how this may contribute to PTSD:

"Tribe: On Homecoming and Belonging"

https://www.amazon.com/Tribe-Homecoming-Belonging-Sebastian-...