As stated, in the original post " There was a recent thread about the high rate of economics majors at Williams and other selective LACs, which made me curious about the overall major distribution at highly selective colleges." The point is showing information about colleges that I find interesting.
I don’t think we can assume the difference in major distribution primarily relates to kids at highly selective private colleges being “smarter.” You mentioned history and philosophy. Are you suggesting that kids at typical public colleges often want to major in history, philosophy, and the like; but they can’t handle the tough major, so they switch out?
I’d expect switching out due to difficulty primarily occurs in math-heavy majors, not history and philosophy. Instead I’d expect that students are less likely to start out in those majors and more likely to start out in majors that have a more clearly defined path to employment related to degree after college. For example, it’s easier for many studies to see the path nursing major → nurse, education major → teacher, or criminal justice major → police officer; than it is to see philosophy major → ???, history major → ???, etc.
Biology is the only major that was top 3 at both highly selective colleges and all colleges. At both types of schools, I expect the bulk of those biology majors are thinking about becoming doctors, rather than applying their biology degree directly. Many students at both types of schools want to be doctors – not just “the very top students at elite schools,” The difference is in addition to the hope to become doctor group, a good portion of students at all colleges also pursue nursing and other health professions that do not require further degrees beyond a bachelors, while this is rare at highly selective, private colleges. One important contributing factor to that rarity is highly selective private colleges rarely offer bachelors degrees in pre-professional health fields and instead focusing more on liberal arts majors.