First published on Chris Bateman's blog, Only A Game in August 2005.

Assuming we have built a game with a core activity that the player enjoys, how do we keep them playing, and what makes them stop? It's all about the rewards the player experiences, and the manner in which those rewards are structured.

What sort of rewards can the player experience? Without attempting to define a taxonomy:

• Currency rewards: the acquisition of a game resource that can be spent represents a fairly universal reward system... giving the player shops to spend currency rewards can be effective, provided there is plenty in the shops to choose from. (Note that the shop can be a 'meta-shop' - it need not be a literal shop in the game world).
• Rank Rewards: like currency rewards, but ratcheted - the player gains benefits from acquiring points towards an eventual step up in rank. The classic example is level in a class and level RPG, although in video games, Elite's (entirely cosmetic) Rank system demonstrates that a Rank reward can motivate even without mechanical benefits. A draw for Type 1 Conqueror and Type 2 Manager players if expressed in verbal terms, but if the 'Rank up' is accompanied by sufficient fanfare its appeal can be more universal.
• Mechanical Rewards: such as increases in stats that the player can feel the effect of. Highly motivating for many players - but the mechanical increases must maintain relevance to the play. Effective for Type 2 Manager and Type 1 Conqueror players in particular.
• Narrative rewards: a little narrative exposition is effective for certain players as a reward. A cut scene can be a bigger reward than dialogue - when used well. But overlong or irrelevant cut scenes quickly become devalued. Effective for Type 3 Wanderer and Type 4 Participant players in particular.
• Emotional rewards: related to the above, but applicable when the player feels they have done something for someone in the game. Animal Crossing's present giving, for instance. A draw for Type 4 Participant players.
• New Toys: anything new that can be experimented with is a 'new toy'. Although primarily a mimicry reward, there may be mechanical benefits of well - a new weapon in an FPS is a new toy with mechanical rewards, for instance. Especially of value to Type 3 Wanderer players.
• New Places: like new toys, new places are a mimicry reward for players driven to explore (a common drive!). Especially of value to Type 3 Wanderer and Type 1 Conqueror players.
• Completeness: perhaps only a drive for the Type 1 Conqueror player (or the Rational player), achieving completeness (chasing 100% for instance) can be a reward in itself.
• Victory: defeating a challenging foe (or a boss) is purely agonistic reward, especially appealing to Type 1 Conqueror players.

The other aspect of how we design rewards is the way the delivery of the reward is structured. Psychologists call these structures contingencies or schedules. I first learned about this from John Hopson's excellent (but nervously delivered) GDC talk Behavioural Game Design, which is also available as an article at Gamasutra. Briefly, psychologists have observed a number of different reward schedules in animals (humans included):

• Fixed Ratio Schedules: these provide rewards after a fixed number of actions. They produce a high level of activity and are easy to understand, but after the reward is achieved, there is a pause. XP in CRPGs is an example - although the gearing of XP systems is exponential, the intent is that the player is constantly moving up to tougher foes, thus keeping a constant ratio of kills to level. Hugely addictive to Type 1 Conqueror players, they can work for any play style - if the rewards are right.
• Variable Ratio Schedules: these provide rewards after a random number of actions - like a slot machine. You keep putting in coins, because at some point it will pay out. These also produce a high rate of activity and interest, but they tend to block exploration - as the player will stick with the reward schedule until it is exhausted, or until they burn out on it. Effective with all play styles - but burnout is always a risk. They are inherently aleatory, and may appeal less to Type 1 Conqueror and Type 4 Participant players.
• Fixed Interval Schedule: that is, a reward is provided after a set amount of time. This provides better control over the rate of reward, and comes with the same post-reward pause as a fixed ratio schedule. Indeed, pauses are inherent to fixed schedules of all kinds. An example is the new items in the shop each day in Animal Crossing - the player comes back on future days to see what's new.
• Variable Interval Schedule: like the variable ratio schedule, this produces a steady rate of activity with no pauses - but its not as intense as the variable ratio schedule, because players quickly learn that their actions are independent of the reward. Good for encouraging a player to come back to certain places in a game, however, if a reward appears in certain places 'at random'. Again, they are aleatory and hence may not appeal to all Type 1 or Type 4 players.

In general, ratio schedules produce high rates of activity - "the more you do, the more you get". Variable schedules produce constant activity - "everything has a chance of reward". When these combine, (variable ratio schedule), the player will eventually burn out. Conversely, fixed schedules create a pause - which needn't be a negative matter. To keep a player's interest in a CRPG, the 'pause' after gaining a level frees the player up from the treadmill of levelling up to go and carry out other housekeeping activities in the game. (If the player levelled up with a variable ratio schedule, they could rapidly get burned out).

These elements - types of reward, and the schedules upon which they are delivered - form a framework which maintains a player's interest in the game they are playing. The more complex the game, the more different rewards and schedules for the delivery of those rewards are required to keep the player involved. A simple game can be built upon a single schedule.

All of this comes to a head in how and why the player stops playing. Pauses allow and encourage quitting - players are constantly evaluating the very next thing they can do in a game, and if their level of interest drops below the draw for another activity, they stop. If this happens through burnout, they may not go back - the gamble with variable ratio contingencies - but if they stopped because of the pause after achieving a sufficiently large reward in a fixed schedule, they will likely come back. In this regard, one must be careful that the rewards themselves maintain at the very least a constant (and at most an escalating) level of reward.

Of course, the best case is that just playing the game is inherently enjoyable to the player - that the core play is its own reward (when the core play is a flow state for the player). Still, even when this is so, the player is likely to stop playing when 'they have seen everything'. This is when multiplayer elements can extend the play window of a game, of course, by providing new rewards - provided the player happens to be motivated by agon (like many Type 1 and Type 2 players).

If one creates a game which is inherently fun for the player, an exponentially structured fixed ratio schedule can be sufficient framework to keep playing - such as the monuments in The New Tetris (which are wholly cosmetic, and equivalent to Rank rewards). For some reason, this structure seems to work better than other schedules for a high level framework. Interval schedules lack the connection with player action, and variable schedules only work until the rewards are exhausted. But diehard players of The New Tetris routinely reset the monuments and start over again, with little loss of interest. Perhaps it is the exponential gearing which drives the appeal, pulling the player forward by gradually increasing the jump to the next reward.

If you have ever wondered why games with poor game mechanics can still entertain players, it is perhaps because many poorly designed games are at least easy to play - and if they provide rewards according to a reliable schedule, they will entertain. As long as they keep providing rewards. Good game mechanics can aid a game by eliminating rough edges and inconsistencies, and some players (those who fit the template of our H1 and H2 clusters in particular; those with access to the Rational temperament) are actively drawn to elegant game mechanics - but it is the delivery of rewards, and not the quality of the game mechanics, which maintains a player's interest.