These studies include experimental studies (where it can be shown that playing violent games actually causes increases in aggression), correlational studies (where long-term relations between game play and real-world aggression can be shown), and longitudinal studies (where changes in children's aggressive behaviors can be demonstrated). For example, in a study of over 400 3rd - 5th graders, those students who played more violent video games early in the school year changed to become more physically aggressive later in the school year, even after statistically controlling for sex, race, total screen time, prior aggression, and other relevant variables. Apparently practice does make perfect.