Why is content knowledge so important to reading? A couple of reasons are obvious: (1) you can’t comprehend if you’re missing some of the vocabulary, and (2) the text might use vocabulary you know, but reference ideas that you don’t know. For example, the sentence “Gosh, it’s January 5th-I’ve got to go get some wheatberries and raisins!” doesn’t use unusual vocabulary words, but it’s not sensible unless you know that the speaker is Armenian, and that a traditional dish for January 6 (Armenian Christmas) is a pudding that includes those ingredients.
But content knowledge serves reading in more subtle ways. A key feature of all writing (and speaking) is that information is omitted. For example, suppose you read the following sentence:
John said ‘look Dave, I would stand in line with you for the tickets, but I’ve used up all my sick days.’
There are two key ideas in the sentence: (1) John wants to stand in line for the tickets but can’t and (2) John has used up his sick days. The second idea is offered as an explanation for the first. But notice that a good deal of information that is necessary for the right interpretation is actually missing from the text. You need to know that (1) people may wait hours in line for tickets to entertainment events; (2) people may use sick days to avoid work even when they are not sick and (3) people are reluctant or unwilling to skip work when they have used all their sick days because their pay may be docked. The writer has omitted this information, gambling that the reader already knows it, and can fill the logical gap in the sentence. If the reader does not have the requisite background knowledge, he or she doesn’t comprehend the sentence.
Writers must omit some information-if they didn’t, writing would be impossibly repetitive and tedious. So readers must bring background knowledge to the task of reading so that they are ready to fill the gaps that writers will leave. Small wonder that students who score poorly on reading tests suddenly look like terrific readers when given a passage on a topic that they know a lot about.
I’ve described just one of the more subtle ways that background knowledge helps reading comprehension. There are others, described here http://www.aft.org/pubs-reports/american_educator/issues/spring06/willingham.htm.