Like many subjects involving human behavior, things are not so quite cut-and-dry.
A suggested reading list for all who are interested:
Anderson, C. A., & Bushman, B. J. (2001). Effects of violent video games on aggressive behavior, aggressive cognition, aggressive affect, physiological arousal, and prosocial behavior: A meta-analytic review of the scientific literature. Psychological Science, 12, 353-359.
Anderson, C. A., & Dill, K. E. (2000). Video games and aggressive thoughts, feelings, and behavior in the laboratory and in life. Journal of Personality and Social Psychology, 78, 772-790.
Anderson, C. A., & Ford, C. M. (1986). Affect of the game player: Short-term effects of highly and mildly aggressive video games. Personality and Social Psychology Bulletin, 12, 390-402.
Anderson, C. A., & Morrow, M. (1995). Competitive aggression without interaction: Effects of competitive versus cooperative instructions on aggressive behavior in video games. Personality and Social Psychology Bulletin, vol. 21, 1020-1030.
Ask, A., Autoustinos, M., & Winefield, A. H. (2000). To kill or not to kill: Competitive aggression in Australian adolescent males during videogame play. In C. van Feilitzen & U. Carlsson (Eds.), Children in the New Media Landscape. Goteborg, Sweden: UNESCO International Clearinghouse on Children and Violence on the Screen.
Ballard, M. E., & Lineberger, R. (1999). Video game violence and confederate gender: Effects on reward and punishment given by college males. Sex Roles, 41, 541-558.
Burke, T., & Burke, K. (1999). Saturday Morning Fever: Growing Up with Cartoon Culture. New York: St. Martin’s.
Calvert, S. L., & Tan, S. (1994). Impact of virtual reality on young adults’ physiological arousal and aggressive thoughts. Journal of Applied Developmental Psychology, 15, 125-139.
Colwell, J., & Payne, J. (2000). Negative correlates of computer game play in adolescents. British Journal of Psychology, 91, 295-310.
Cooper, J., & Mackie, D. (1986). Video games and aggression in children. Journal of Applied Social Psychology,16, 726-744.
Csikszentmihalyi, M. (1990). The flow experience. San Francisco: Jossey-Bass.
Cumberbatch, G., Maguire, A., & Woods, S. (1993). Children and video games: An exploratory study. Communications Research, Aston University, Birmingham, U.K.
Dill, K. & Dill, J. (1998). Video game violence: A review of the empirical literature. Aggression and Violent Behavior, 3, 407-428.
Durkin, K. (1995). Computer Games, Their Effects on Young People: A Review. Office of Film & Literature Classification. Sydney. <255 Elizabeth Street, Sydney NSW 2000, Australia>
Fowles, J. (1999). The Case for Television Violence. Thousand Oaks CA: Sage
Funk, J. B. (1993). Reevaluating the impact of video games. Clinical Pediatrics, Feb., 86-90.
Funk, J. B., & Buchman, D. D. (1995). Video game controversies. Pediatric Annals, vol. 24, 91-96.
Garvey, C. (1991). Play. (2nd ed.) Cambridge, MA: Harvard University Press.
Goldstein, J. (1999). The attractions of violent entertainment. Media Psychology, 1, 271-282.
Goldstein, J. (1998). Immortal Kombat: The attractions of video games with violent themes. In J. Goldstein (Ed.), Why We Watch: The Attractions of Violent Entertainment. New York: Oxford University Press.
Goldstein, J. (1995). Aggressive toy play. In A. D. Pellegrini (Ed.), The Future of Play Theory. Albany: State University of New York Press.
Grainger, J., & Dijkstra, A. F. J. (1996). Visual word recognition: Models and experiments. In A. F. J. Dijkstra & K. J. M. de Smedt (Eds.), Computational Psycholinguistics. London: Taylor & Francis.
Graybill, D., Strawniak, M., Hunter, T., & O’Leary, M. (1987). Effects of playing vs. observing violent vs. non-violent video games on children’s aggression. Psychology: A Quarterly Journal of Human Behavior, 24, 1-8.
Griffiths, M. (1999). Violent video games and aggression: A review of the literature. Aggression & Violent Behavior, 4, 203-212.
Griffiths, M. (1997). Video games and aggression. The Psychologist, Sept., 397-401.
Grodal, T. (2000). Video games and the pleasures of control. In D. Zillmann & P. Vorderer (Eds.), Media Entertainment. Mahwah, NJ: Erlbaum.
Gunter, B. (1998). The Effects of Video Games on Children: The Myth Unmasked. Sheffield, UK: Sheffield Academic Press.
Holmes, R. M. & Pellegrini, A. D. (1999). Children’s social behavior during video game play with aggressive and non-aggressive themes. Paper presented at International Toy Research conference. Halmstad, Sweden.
Kellerman, J. (1999). Savage Spawn: Reflections on Violent Children. New York: Ballantine.
Kirsh, S. J. (1998). Seeing the world through Mortal Kombat-colored glasses: Violent video games and the development of a short-term hostile attribution bias. Childhood, 5, 177-184.
McCauley, R. C. (1998). When violence is not attractive. In J. Goldstein (Ed.), Why We Watch: The Attractions of Violent Entertainment. New York: Oxford.
Provenzo, E. F. (1991). Video Kids: Making Sense of Nintendo. Cambridge MA: Harvard University Press.
Roe, K., & D. Muijs. (1998). Children and computer games: A profile of the heavy user. European Journal of Communication, 13, 181-200.
Sacher, W. (1993). Jugendgefährdung durch Video- und Computerspiele?
Zeitschrift für Pädagogik, 39, 313-333.
Schutte, N. S., Malouff, J. M., Post-Gorden, J. C., & Rodasta, A. L. (1988). Effects of playing video games on children's aggressive and other behaviors. Journal of Applied Social Psychology, 18, 454-460.
Scott, D. (1995). The effects of video games on feelings of aggression. Journal of Psychology, 129, 121-132.
Sherry, J. (2001). The effects of violent video game on aggression. Human Communication Research, 27, 409-431.
Sherry, J., Lucas, K., Rechtsteiner, S., Brooks, C., & Wilson, B. (2001). Video game uses and gratifications as predictors of use and game preference. Paper presented at International Communication Association.
Silvern, S. B., Lang, M. K., & Williamson, P. A. (1987). Social impact of video game play. In G. A. Fine (Ed.), Meaningful Play, Playful Meaning. Champaign Il: Human Kinetics Press.
Silvern, S. B., & Williamson, P. A. (1987). The effects of video game play on young children’s aggression, fantasy, and prosocial behavior. Journal of Applied Developmental Psychology, 8, 453-462.
Sorensen Holmes, B., & Jessen C. (2000). It isn’t real: Children, computer games, violence and reality. In C. van Feilitzen & U. Carlsson (Eds.), Children in the New Media Landscape. Goteborg, Sweden: UNESCO International Clearinghouse on Children and Violence on the Screen.
van Schie, E. G. M., & Wiegman, O. (1997). Children and video games: Leisure activities, aggression, social integration, and school performance. Journal of Applied Social Psychology, 27, 1175-1194.
Wiegman, O., van Schie, E. G. M., & Modde, J. M. (1997). Computerspelletjes en hun effecten. Justitie Verkenningen, 23, 21-34.
Wingrove, J., & Bond, A. J. (1998). Angry reactions to failure on a cooperative computer game: The effect of trait hostility, behavioral inhibition, and behavioral activation. Aggressive Behavior, 24, 27-36.
Winkel, M., D. M. Novak, & H. Hopson. (1987). Personality factors, subject gender, and the effects of aggressive video games on aggression in adolescents. Journal of Research in Personality, 21, 211-223.
Zillmann, D. (1998). The psychology of the attraction of violent entertainment. In J. Goldstein (Ed.), Why We Watch: The Attractions of Violent Entertainment. New York: Oxford University Press.
http://culturalpolicy.uchicago.edu/conf2001/papers/goldstein.html
And what "effect" are we talking about? Aggression is the most common studied effect on gamers...but an interesting thing that I've yet to see is how the aggressive level of gamers compared to people who watch Sports on tv or porno. Just saying that an activity increases aggression really tells us nothing in of itself. It's just a meaningless fact in isolation until seen in context to other human activities.
Heck, playing chess increases my aggression level. I've seen people weirded out over games of "go fish". I think it's always more of a question of the inherent aggression levels of a person than any activity they're involved in.