First of all, it's technically MD/PhD.
As a freshman in college, you do not need to decide if you want to do MD/PhD "soon." You should try to get into meaningful scientific, hypothesis-driven research as soon as possible, make good connections with your mentors that will result in strong LORs, and potentially even get a publication. THEN you can decide, if you like research enough, to pursue MD/PhD.
Just from my observation from the MD/PhD program at my school, the majority of the candidates majored/did research in BioE or Bio/Molecular Bio. A few majored in Chem/ChemE/CS/EE (aka probably not medically-related at all). I can't speak to whether or not psych research would be beneficial for someone applying MD/PhD because I'm not extremely familiar with the admissions criteria, but just from one school, it seems like bio/medically-related research is most common.