At one of their online info sessions someone asked this question, and I believe their response was that citations are not needed; unless, of course, you are using ideas that are not your own. Personally, I included citations because: i) I made reference to studies and theories that related to my topic of interest, and; ii) it asks to base our response on academic literature. Of course, this is all dependent on your writing style, so I doubt they will look at your response unfavourably if you don't use references. I hope this helps and good luck with the personal statement