To what degree does your major affect admissions?

Answered

So something to understand about the UCs is that they're different in a lot of ways from most other colleges. At MOST colleges, your major does not significantly affect admissions chances—colleges will look at how well your profile and portfolio/application matches the thing you're interested in studying and judge whether you're a good fit based on that, but they won't limit the number of seats available for a given major. "Impacted" majors are a thing specific to the UCs. So this isn't as much of a consideration outside those schools, and anywhere beyond the UCs, you should absolutely apply for biology if it's what you'd prefer to study.

Regarding the UCs, my general response here is that you should go for the thing you want to study and not try to "play it safe" with a major you think will be easier to get into, because that can have unintended consequences down the line. The main reason I'd say that here is that, if for whatever reason you decided not to aim for medical school afterwards, or even if you wanted to do something career-wise for a couple of years before going, psychology is a less versatile major than biology. Unless you're specifically interested in psychiatry (which it doesn't sound like, but would change my answer), human biology would also give you a better basis in the subjects you'll need to be successful in medical school—or at least, to get the same foundation as a psychology major, you'd need to take a lot of courses beyond just your requirements. With that in mind, if biology is what you want to study, I think you should aim for that despite it possibly making it a little harder to get into some of the UCs.