What happens, for instance, as you add more participants to a carefully-designed environment? The online role-playing game World of Warcraft (WoW) provides an intriguing example. More than 11.5 million people around the world now play World of Warcraft. Performance in the game is measured by experience points, which are awarded to players as they successfully address progressively more difficult challenges. It takes roughly 150 hours of accumulated game play to earn the first 2 million experience points but players on average are able to earn another 8 million experience points in the next 150 hours of accumulated game play. Even though, within the game, experience points become more difficult to acquire as you advance, World of Warcraft players are improving their performance four times faster as they continue to play the game.
via blogs.hbr.org