For US college admissions (especially for more competitive or elite colleges), does it matter whether your spike is based on your academic interests and what you plan to major in? Or is it better for it to be based on your hobbies and other interests outside of school? The majority of my current activities are related to what I want to study (global health, medicine, biomedical sciences), so I was wondering if I should also do more activities related to my hobbies (art, literature) or try to maybe combine the two? (I’m going into 10th grade next year)
I think you are asking the wrong question.
First of all, a spike is something that sets you uniquely apart from the rest of the applicants in the pool you part of. So if you are an Int'l Student applying for a pre-med track (BIO, CHEM, STEM major) at a top American college, you will be in a pool of applicants who have all an overlap of the same interests and activities. Some of these overlaps will be internships, research opportunities, volunteering, inventing medical devices, programming, etc. If the group as a whole has a lot of the same ECs then these are not "Spikes" but common attributes and ECs Overlaps. If there are 10,000 Int'l applicants who are applying to UPenn and many of the applicants have the same activities, then they all look the same to the admissions office. So while all these students think they have spikes, what they have are common traits.
My second point is that at top colleges like Stanford and Harvard, there is something they are looking for called Intellectual Curiosity or Intellectual Vitality. While this could be a "spike", most often it is evidence that the applicant has looked beyond the traditional classroom to pique his/her/their interest in the topic. For instance, if you love astronomy but your school doesn't teach that, colleges would be impressed if you obtained college credit in an Astronomy Course and have spent 100 hours at a local observatory working on a research project, and perhaps have created a youtube channel about sharing your passion for astronomy with your 10000 followers.
The last point I want to make is that a "spike" might start off as a hobby like art or literature but for it to be a "spike" you have to accomplish a lot with your passion and consistently score goals across the finish line. A spike is not doodling in your art portfolio or reading 100 books. A spike in art might be creating a series of highly effective graphic art posters for various non-profit agencies to effectively communicate their fight and cause, it's using your voice and talent to make a difference in the world and make it a better place. A spike is not reading a lot of books. A literature spike might be writing your own volume of short stories or poetry and getting them published and translated into 10 languages.
If there are 1000 carp in a pond and Harvard wants to scoop up 34, they are going to ignore all the carp that look alike and have the same attributes. They are most interested in the ones that stand out because they are the best examples of being a carp. So you have 2 1/2 years to figure out how to be the best version of yourself. This has nothing to do with what other people are doing or what they think needs to be listed as an accomplishment in their application file.
Good luck.
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Thank you for this! I understand the concept of spikes a lot better now. Best of luck to you too.