The previous section mentioned a number of factors involved in reading comprehension; these and their antecedents can be seen as the causes or sources of reading comprehension (see Figure 2). As shown in the Figure, vocabulary knowledge (Wagner et al., 2007) and prior knowledge are thought to be contributors to listening comprehension, though both and many other factors shown are also related to verbal intelligence. It is difficult to see how readers can understand a text if there are too many unknown words or concepts.