Oh boy. I'm going to dispense with the overly-sarcastic opening I was going to use and just address your "arguments," such as they are, one by one, but not in the order you presented them.
If video games have potentially thousands of effects, why would violence be strictly of concern?
Okay, I'm going to try and explain this in the simplest terms possible. The person I was replying to claimed that the study was biased because it did not simply research "What are the effects of video games on children." I said this was absurd because one study cannot capture all of these effects. I specifically did NOT say that every singe potential effect that anyone could possibly imagine is equally worthy of its own study. There are a wide variety of reasons that a researcher would choose one potential effect over another for study - personal bias is one, yes. But others include (but are not limited to) the existence of previous research showing similar effects in a related field, and testing a theoretical framework that would suggest the presence or absence of a particular effect. These theoretical frameworks are generally based on such previous work, but in their infancy might also include some logical thinking and common sense. Which brings us to...
Would testing the influence of video games on the rates of homosexuality make for a good study (cause you know Tetris does have homoerotic undertones)? How about likelihood of video game players to favor volunteer work?
How many violent video games (remember, this study is only on VIOLENT video games, not ALL video games) feature violence, or other factors that have been related to violence in previous studies? How many feature homosexuality, or factors that have been related to violence in other studies? How many feature volunteerism, or other factors that have been related to volunteerism in other studies? I'm guessing the first number will be much higher than the other two. This does not, of course, mean that violent video games cause or are even correlated with violence - but it does mean that one could probably make a sound theoretical argument for studying it. There are probably people out there studying video games and sexuality as well as video games and volunteerism. I myself study video games (and television) and the understanding of scientific practice. You are making a huge mistake in assuming that just because some people study violence, doesn't mean nobody is studying the possible positive effects.
Couldn't violence be completely mitigated by one of the other effects? Couldn't a combination of other effects lead to violence (not specific to video games)?
It's entirely possible. Why don't you go read the 130 studies involved and see if any of them controlled for any of these factors? I'll bet at least a few of them did. Not saying they all do, or do it as well as they could, but this is what these people do for a living.
What you are telling me is the questions the social sciences are conditioned to ask reflect less any discernible data (quite honestly if the social sciences can't differentiate through several different possible responses, it is immature science that should know better than to come to an conclusions on this matter) but merely reinforce cultural norms and stereotypes by the very nature of the questions it asks.
No, that is bullshit that you made up yourself. Please show me where I said that social scientists do not use existing data/studies to formulate their research questions.