Most of the classes here are difficult because of the nature of the students who take them. If the class is an easy course, chances are that there are quite a few over-qualified students who are taking the credit to boost their GPA - which increases the competition. Higher level courses, which are usually restricted to upperclassmen and students who are specifically majoring in that department, are filled with hard-working, competitive students who are taking the class because they have to or because they absolutely love the material. Since Yale requires students to take classes in different subjects (according to their liberal arts education approach), most students will get equal experiences of getting good grades easily and having to struggle to go above the average.
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