What factors and techniques contribute to making a user experience addictive?
I ask this in the context of websites, games and other software. What makes people want to keep coming back for more?
Most of the web games base their "addiction" on simple behavioral methods (yes, like the mouse in a Skinner box).
Game like Farmville or Mafia Wars wars use reinforcement, Doom emotional design.
Another classic category of games with high addiction levels are RPGs with their online version MMORPG which have also a strong base of catharsis (like the Psychodrama)
This kind of trick could easily be transposed into other activities not related to gaming purposes.
So that's all folks? Nope, there are plenty of factors that can determine the addiction level but these in my opinion are the most evident.
I want to add also a great book:
It's a popular book by Robert Cialdini and it's a good book to know more about psychological bases of marketing&co.
There's some interesting research done on the concept of Flow, or immersion. Mihaly Csikszentmihalyi wrote a book called Flow, and in that he discusses how flow "is a mental state of operation in which the person is fully immersed in what he or she is doing by a feeling of energized focus, full involvement, and success in the process of the activity". Factors such as challenge, skill level, and even personality or character affect how one can achieve this state.
What makes something addictive generally falls into one of 5 categories:
One technique, already mentioned, is using reputation systems. There's a great presentation on this topic from Bryce Glass (Yahoo).
Also, there are different reward systems that can be applied, read more about them on the Casual game design blog.
If addictive is loosely defined as "an essential service" or "somewhere I just have to be" then imho these are the factors (divorced from the user, which is of course a dumb thing to do, but for illustrative purposes) :
Aesthetics: the feel of a site conveyed through the graphic design can make many users, or a certain type of user, feel at home....comfortable...a nice place to 'be'.
Goals: what goals are the user trying to achieve? If you're supplying them, there's a good chance (in conjunction with everything else) that it will encourage that addictive nature that you're trying to encourage. For example, I find UXMatters addictive - freshly updated material, good aesthetics, good community...
Community: if the content is to be generated by the community, having a good community will also help generate addicitivity because fresh, interesting content is generated that means that the user has to keep returning to get their fresh fix of info goodness. Anyone can build a sexy site...but it takes great users to make it exceptional (imo)!
Personalisation: ensuring that the system or the user generates content that is directly relevant to the user...as well as aesthetic skins to enable the user to tailor his own addiction (more toys = more retention, but not always). Essentially a personal filter.
I can't think of any others at the moment...but my gut feeling is that with the correct amount of personalisation (and/or customisation) AND content AND community that you could end up with a lovely site with more addictive potential than sugar coated crack? :)
e2a: Specifically regarding games, making them addictive is all down to gameplay. Way way too many games publishers go for the 'ooooh shiny' approach and forget about the basics of gameplay. Get the gameplay right and everything else follows. Not directly relevant to UX Exchange imo....but check out Angry Birds on the iPhone if you want the very definition of addictive gaming!
Stephen Anderson's book Seductive Interaction Design has good coverage on all the abovementioned aspects, if you'd like a deeper dive.
It's when the person achieves flow with the system.
In my positive psychology class, I was introduced to Mihály Csíkszentmihályi's concept of flow. We made a quick survey and found out that our peers achieved flow in activities like gaming and surfing the Internet. I don't remember if there were specific games or websites mentioned, though.
To sum it up, the system should create a flow and it shouldn't be easy to break.
I recently did some research on this and talked to Dr. Clayton Lewis (computer Scientist in Residence @ CU Boulder). Much of my answer comes from the copy of Engagement Analysis he gave me. Another (healthier and more appealing) way to describe addiction is Engagement. Factors that Encourage Engagement
Caveat:remember that Engagement only works if you are merely surfacing the intrinsic motivations the "player" already has. If you hate your job at the factory but care about the people that use the cars you build then dangling money in front of you for building a better car is more expensive and less effective than illuminating the impact you have on the users. E.g., have them come to the factory for a tour.
Having studied addiction in school it seems like there is a little misunderstanding of what it is, and how it's different then the development of a habit.
What makes a web/game addictive is anything that causes a chemical change in the brain, and withdrawal symptoms when the web/game is removed. If there is no withdrawal then the clinical definition of addiction has not been met.
To simplify this answer, I'm going to say that in most web/game addictions there is a connection between the stimulus that triggers the release of dopamine.
While a lot of the answers here are very good ideas. They are not exactly on the mark for addiction. A lot of the answers relate more to "shaping behavior" and that doesn't cause addiction. Porn websites for example can be addictive, and they employ none of the techniques you'd find in a well designed video game.
Here are my points for a designed web/game to be addictive.
Finally, addictive does not mean good design or success. It just means people find it hard to stop, and that doesn't always bring those who created the thing success. It's much better to have a well designed product that solves a problem and there is a need for it, then something that is just addictive.