The researchers suggest that "the haptic and tactile feedback of a Kindle does not provide the same support for mental reconstruction of a story as a print pocket book does".
"When you read on paper you can sense with your fingers a pile of pages on the left growing, and shrinking on the right," said Mangen. "You have the tactile sense of progress, in addition to the visual ... [The differences for Kindle readers] might have something to do with the fact that the fixity of a text on paper, and this very gradual unfolding of paper as you progress through a story, is some kind of sensory offload, supporting the visual sense of progress when you're reading. Perhaps this somehow aids the reader, providing more fixity and solidity to the reader's sense of unfolding and progress of the text, and hence the story."
Mangen also pointed to a paper published last year, which gave 72 Norwegian 10th-graders texts to read in print, or in PDF on a computer screen, followed by comprehension tests. She and her fellow researchers found that "students who read texts in print scored significantly better on the reading comprehension test than students who read the texts digitally".