Google conducted a research on the way the eyes of the user move around the SERP. It is important for Google to know where eyeballs go on a search result page. In a post in The Oficial Google Blog about the results of the study, User Experience Researchers Anne Aula and Kerry Rodden have the following words: "Based on eye-tracking studies, we know that people tend to scan the search results in order. They start from the first result and continue down the list until they find a result they consider helpful and click it — or until they decide to refine their query. The heatmap below shows the activity of 34 usability study participants scanning a typical Google results page. The darker the pattern, the more time they spent looking at that part of the page. This pattern suggests that the order in which Google returned the results was successful; most users found what they were looking for among the first two results and they never needed to go further down the page."
Google is conducting similar eye tracking tests with image search and Google News.