The thing that is interesting about most of the MMO programs on-line today is that they use behavioral conditioning as part of their effort to keep players coming back to the game by way of intermittent reinforcement schedules that condition a player to continue to seek more drops, loots, etc.
Of course, this maximizes revenue for the MMO producer.
What is significant about such conditioning is that it reaches the psychology of the player, even when they are not in the game, by creating a DESIRE to play more. So for people who have a tendency to addictive behavior this conditioning is more effective.
I am certain that MMO designers used behavioral psychologists to set up their reinforcement schedules to maximize this effect. I have seen the same sort of reinforcement scheduling when working with animals in behavioral conditioning laboratories; what works on lab rats works on us, too.
It isn't a trivial matter; the Internet is full of stories about players who took their on-line experience to unhealthy extremes.