Google's search options sidebar includes a feature that was only available in the advanced search page: "filtering results by reading level." When enabled, Google will classify search results based on the complexity of the text. You can restrict the results to "basic" pages, "intermediate" pages and "advanced" pages, which are mostly scholarly articles.
"Sometimes you may want to limit your search results to a specific reading level. For instance, a junior high school teacher looking for content for her students or a second-language learner might want web pages written at a basic reading level. A scientist searching for the latest findings from the experts may want to limit results to those at advanced reading levels," explains Google.