He has served as an expert witness for the state of Pennsylvania for his work studying the existence of a relationship between violence in video games and the real world. In his work, Markey has found that aggressive thoughts do increase immediately following time playing violent games. These findings have been artificially expanded by some to imply that violent games are a risk factor. Markey cautions against this impulse, especially since over the past fifteen years, mature game sales have increased while violent crime rates have dropped. Furthermore, the link between a perpetrator's terrible actions and history having played video games is a useless data point, since 97 percent of adolescents play.