Read the whole thing at USA Today.
Researchers at several universities reviewed about 8,100 images in 296 children's books. The books were all Caldecott Medal winners and honorees from 1938 to 2008. The Caldecott awards are given annually to the artist of the most distinguished American picture book for children.
Researchers categorized images as containing either a natural environment, such as a jungle or forest; a built environment, such as a house, school or office; or a modified environment, such as a mowed lawn, park or farm field. They also identified wild and domestic animals.