“Some poor people overseas are physically retarded, due to lack of micronutrients like iron or iodine. But a lot just don’t do much abstract thinking. This doesn’t mean their offspring wouldn’t be able to, but they don’t.
“So they score low on IQ tests, and perform poorly at real-world tasks that require higher IQs.
“I’ve heard of an anthropologist who has spent years with the Bushmen of the Kalahari Desert who can’t recall ever hearing a conversation among them about something abstract.
“An extreme example of concrete, here-and-now, nonabstract thinking appears to be a tribe in the Amazon called the Pirahã. They’ve been known to Portuguese-speakers since the 1700s, but they’ve successfully resisted being assimilated by their remarkable lack of curiosity about and contempt for anything outside their own time and place.”