I am looking for some clarification for whether or not your intended major affects your acceptance into highly ranked schools. I intend to major in philosophy, and I was wondering if, since this is (as far as I know) an unpopular major, this will help me a little.
If it is an admit to overall university, then no, it likely won’t change anything. But if the school is an admit-by-college or admit-by-major, then there will be weight added to what you intend to do.
For example, if you are admitted as a philosophy major at Carnegie Mellon and request to change that to computer science, that will not happen.
It does vary a bit from place to place, but at most universities you are accepted into a school, not a major. So you would apply to the liberal arts school, the engineering school, the business school etc. Within the school you are free to change majors but it is often very difficult to transfer from a school that is less competitive admissions-wise into a school that it is more difficult to gain admission to (see example above). Colleges are wise to the game of students applying for a less competitive school and then trying to do an internal transfer and most discourage the practice by making it exceedingly difficult to do so.
Within a school, it does not generally matter what major you choose in terms of admission chances. Admissions officers are also wise to the idea of someone applying with an “unusual” major and then switching it once they arrive on campus. I’ve heard admissions officers say that the intended major is typically not a factor as: (a) about half of the students apply undecided and (b) about half of the students that come in with a major end up changing the major while they are in college.
A possible exception might be if an applicant has an unusual major as well as a proven and long standing track record of having taken classes/done work in that area.
Again, no one answer will fit every school but this is my general understanding.