In an interesting psychological study, Nicholas Yee, a student at Havefort College, polled over 4000 EverQuest players to discover things about interpersonal relationships, addiction, and gender issues related to Sony's MMORPG. Among the questions asked were such elements as "Why does EQ appeal to you?", "Would you hack the game if you could?", and "Has your real life relationships suffered because of EQ?"
The results of the study, as well as the methods, materials, and demographic data can be found at Mr. Yee's website. And if you're wondering, the majority of those surveyed were either neutral or in favor of role-playing relationships in the game.