I'm a Literature and Biology double major. Majoring in the humanities is not for slackers; after intro classes, literature and history or whatever else isn't a cakewalk. Furthermore, humanities classes allow for a broad base of knowledge, better communication and writing skills, better people skills, and an understanding of the world you live in. It allows you to see outside the science of medicine and into the art of it: the art of healing, dealing with people, developing yourself as a mature person. Also, humanities keep you balenced, because you're gonna have to take science classes as a pre-med anyway, and you might as well take humanities b/c you won't have any in med school. I'd much rather be able to tell my interviewer about how John Donne's poetry conveys the sense of helplessness and weakness of the persona as he lies dying, rather than constantly have to refer to how much I loved doing the xyz experiment in the endless lab courses I took. Don't get me wrong, I love science and love research even more, but with med school balence is key.
P.S. Psych is a science.