Do you think your moods are controlled by your brain chemistry or that your brain chemistry dictates your moods? Do you believe people are born with particular emotional temperaments or that they are primarily shaped by environmental factors?
Embrace the power of "and".
(I know that these Writers Block questions are supposed to stimulate writing, but so many of them are yes/no or "this/that" questions or I'm tempted to start challenging myself to answer them in as few characters as possible. If this one were part of the challenge, my answer would have been "and".)