We tend to blame cleanliness but we may not be right; my gut feeling is that we are not.

The fault may quite easily be with carbon pollution, which exists more in cities and in the first world than in rural areas, and which has now been fingered as causing huge loss of intelligence.

It may also quite easily be the extremely indoor lifestyle of first world and city children, which causes a gene for nearsightedness to express in those with the genetic tendency, if they don't play outside a lot as children.