Bartle's Taxonomy was the earliest attempt to break down player psychology in a multiplayer environment. Richard Bartle, who created the first MUD in 1978, interviewed the players of his games about why they played. Their responses fit into four categories, which we now call Achievers, Explorers, Socializers, and Killers. Achievers focus on in-game goals like getting high scores or collecting gold. Explorers seek to discover new locations on the map or new ways to use the mechanics. Socializers come to meet people, often organizing guilds or collecting on social forums. Killers seek to dominate other players, usually by killing them in PvP. Bartle went further than creating these four categories, however: he also mapped them to a graph with Action-Interaction on one axis and Player-World on the other. This simple graph helps developers evaluate new content: what category does it fall into, and therefore what type of gameplay does it encourage?