The earliest I've managed to trace the phrase "The closer you get to humans, the worse the science gets" is a blog comment by David Marjanović July 13 2007. He claimed it was a proverb. Someone else in the thread then claimed "that's pretty much like a folk heuristic version of Walter M. Elsasser's 'Reflections on a Theory of Organisms: Holism in Biology'". I've been asking around for a better source, but it certainly doesn't seem to be something Elsasser said in those words. Marjanović is fond of it.

Edit: Found an earlier rendition, "Scienticity is everywhere lower where the subject is closer." - from Donald Black. "Dreams of pure sociology." Sociological Theory 18:3 November 2000.