From "Reading Comprehension Requires Knowledge—
"I vividly remember when I first learned about the severe limits of human working memory and their importance in communication. It was in a wonderful book called The Psychology of Communication by the distinguished cognitive scientist George A. Miller.(5) The second chapter was one of the most famous articles ever written in the field of psychology, 'The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information.' The 'magical number seven' turned out to be the approximate number of items (whether simple facts, or numbers, or words representing complex concepts) that you can hold in your conscious mind at one time before they start evaporating into oblivion. This 'magical number seven' is a limitation that (with some variation) afflicts everyone—including geniuses...."
5 Miller, G.A. (1969). The Psychology of Communication: seven essays. Baltimore, Md.: Penguin Books.