It looks like there is not evidence to support this claim, but I wanted to add that margin trading is the biggest reason that any exchange trading on their own books is potentially a conflict of interest. In margin trading if you can see all the other traders' positions you have an advantage akin to being able to see everyone's hand in a game of poker. You can basically print money by knowing exactly how much it would take to make someone liquidate into your orders, or just direct the price slowly in your favor by taking the least crowded position.