In order to improve the understanding of media violence effects, it is crucial to extend knowledge about factors that threaten the validity of such effects in empirical research. Research artifacts can be expected when participants are (a) aware of a scientist’s hypothesis, (b) motivated to confirm or disconfirm the hypothesis, and (c) capable of manipulating their responses in line with their motivation. Based on social identity theory (SIT) and self-categorization theory (SCT), we assumed that identifying with the social group of video game players would provide a motivation to disconfirm the “violent video games increase aggression” hypothesis. We further assumed that the use of nontransparent aggression measures and cover stories would prevent research artifacts. Our results showed that highly identified (compared to lowly identified) players of video games reported less aggression on a transparent aggression measure but not on a nontransparent aggression measure. However, providing participants with a cover story did not prevent hypothesis awareness nor eliminate hypothesis-disconfirming response patterns. These results provide empirical support for the ideas that (a) motivational factors may contribute to a biased estimation of media violence effects, (b) cover stories may not always be effective, and (c) the use of nontransparent aggression measures can provide a valid methodological approach for avoiding biases in media effects research.