I'm surprised no one has mentioned this yet. If you are going to do this experiment, it needs to be _double-blind_. Meaning PG cannot know which username his post is going to show up under until after he has written it. Same goes for all the comments he writes (if you're going to measure those too).

If PG knows before hand what name his content is going to show up under there is a high likelihood of a bias on his part which he would be _unable_ to ignore even if he consciously tried. This is standard psych experiment stuff.