Knowledge is Important for Reading Comprehension
I cannot emphasize enough the importance of being able to read well. Reading is so central to my life, in my leisure time, and in how I pursue ongoing professional development and lifelong learning. It is equally crucial for our students, for them to enjoy literature, pursue further education, and access information online.
Because reading is important, knowledge is important, as knowledge improves reading comprehension. In one study, students’ comprehension of a passage on baseball was correlated, not with previous identification of them as poor or strong readers, but with their knowledge of baseball. Another study looked at how students’ background knowledge on birds affected their reading comprehension of a storybook featuring birds; the difference between students with more knowledge and the students with less disappeared when the storybook was about a made-up creatures called wugs. Simply put, it is easier to read books and articles on topics for which you already possess knowledge.
Literacy is not just a matter of decoding the strings of letters that makes up words or the meaning of each word in a sequence. It is a matter of decoding context: the surrounding matrix of things referred to in the text and things implied by it.
You’ve likely experienced this yourself, when you are reading about a topic with which you are unfamiliar. I certainly have, such as when I delved into Empires of the Word by Nicholas Ostler. I never studied ancient history, either independently or in my history degree, and found the book to be confusing and overwhelming in the amount of information I was trying to absorb. I abandoned the book, with the intention of returning to it later when I’ve improved my understanding of ancient history. My lack of knowledge was a barrier to my comprehension of the book.
As Willingham explains, there are several reasons why knowledge improves reading comprehension. First, there is vocabulary knowledge. Comprehension is better if students know more words, preferably at least 95% of the words in the text. As well, we can learn words more effectively from context when we are already familiar with the content. Secondly, authors do not state everything in their writing. Some things are implied, left as gaps for the reader to fill in using inferences; background knowledge equips readers to make those inferences. Take the example used by E.D. Hirsch: “All men are mortal, so Socrates is mortal.” The information left out is that Socrates is a man. This is a simple example, but gaps left by writers can be found again and again in texts. Anthony Radice uses a passage from a physics website to demonstrate the importance of background knowledge; personally, I know most of the words in the passage, but still do not understand it.
Authors also make references in their writing, to support their points. I am currently reading The Story of Philosophy by Will Durant (and quite enjoy it, when I have the attention span). The book was originally published in 1926, and is sprinkled with historical references in its illumination of the lives of philosophers. Durant comments on how there “rages a sort of Homeric question” for the authorship of the works of Aristotle ; he mentions an unnamed Corsican lieutenant who tried to rule Europe when discussing Plato’s concept of justice . If I didn’t have the background knowledge to understand these references, I would struggle to understand the points Durant is trying to make about these philosophers. I also find that there is a degree of delight in connecting the dots and getting the reference; it’s like finding an Easter egg in a video game.
If my students, in the future, want to read The Story of Philosophy, Charles Dickens, or any other book, I want them to be able to do so. I want that text to be accessible. I want them to be able to read whatever they want to read. Knowledge will help them do that.
Knowledge is Necessary for the Development of Concepts and Mental Schema
Maybe you don’t care much about students having knowledge about Ancient Rome’s republic, the Tokugawa shogunate, or the Amazon River, but you want them to understand different forms of government, how economies change and develop, and the relationship between people and their environment. 
Well, the knowledge helps with those understandings you seek to achieve.
First, students need the vocabulary for these ideas. The Big History Project recognizes the importance of having words for these concepts, in a history program orientated around big ideas: the importance of having labels for ideas. The medium-sized ideas, such as nomads, paleolithic and specialization, are needed to access the big ideas, like the rise of civilization. These ideas are language based, and students need this language to understand and discuss them, both within and outside the classroom.
It’s about creating a network of knowledge in students’ minds. If you want students to understand the relationship between humans and the environment, then you can start off with how early civilizations relied on their environment to flourish, by building their cities near rivers, by following the seasons for their harvest, but also shaped their environment to their needs through constructing levees and irrigation canals. You can show how Ancient Greece grew, not as a country or empire, but a disjointed and fractious collection of city-states, separated by islands and mountains, but then in turn used the Mediterranean Sea to support trade and create colonies. Later, can students study at the island of Japan, limited in its own resources; it initially chooses isolation and self-sufficiency, then trade and open borders, then empire and conquest. When students, then, see the sinking islands of Kiribati and Tuvalu, the salty soil of Bangladesh, the devastation wrought by hurricanes, it activates a network of knowledge on how people interact with their environment. To trim knowledge away from the curriculum weakens this network.
Michael Fordham pictures this as a knowledge party in his head, a web of knowledge that is summoned around ideas such as “middle class”: London coffee houses, Phileas Fogg and Charles Darwin. For me, “revolution” summons French, American, Industrial, Russian, Haitian and Quiet, all with the common idea of change, but with diverse processes and outcomes.
A focus on building schema also helps to show how a knowledge-rich curriculum is not simply about memorizing facts. For history, for example, it requires identifying big ideas and important themes, and then identifying the knowledge needed to develop those ideas. For geography, the focus would be on key concepts, and examples to support those concepts. Knowledge makes these abstract ideas concrete and helps build stories to make these ideas memorable.
Finally, knowledge gives students concrete examples to use when discussing big ideas. If you want students to discuss the best form of government, having learned about the ancient Athenian democracy, the various emperors of China, the best and worst dictators of the 20th century, and the limitations of modern democracy, they will have a rich body of knowledge to draw on to support their answer, rather than speaking broadly and in hypotheticals.
Helping students understand these concepts is a worthy goal, but needs to be done through the development of knowledge.
 The point being that, just as we are unsure about who wrote Homer’s epic poems, we are unsure who wrote the works of Aristotle.
 The Corsican lieutenant being Napoleon.
 I’ll be using history and geography examples here, as that is my subject specialty and these examples most easily come to mind.