Generated controversy upon it's release for its pioneering use of "jiggle physics", whereby female characters' (sizeable) breasts are modeled to react dynamically to any forces applied to them. In other words, the characters' breasts bounce around whenever they move about. Although the "Dead or Alive" series has since become synonymous with jiggle physics, by 2004 numerous other titles had incorporated the system, including Mortal Kombat: Deadly Alliance (2002), "Outlaw Voleyball" (2003) and even The Sims 2 (2004)