Who Studies The Most?

This NY Times article entitled "Would Be Engineers Hit The Books the Hardest, A Study Finds" discusses the study habits of undergraduate students. Findings from the National Survey of Student Engagement finds that, in descending order, studying is highest in physical sciences, biological sciences, arts and humanities, education and social sciences. The study also breaks down data by type of school. What other factors would influence this? Is it that the major requires more studying, or that people who choose those types of majors have more or less time to study?