Feedback Structures in Games
The structure of a game's internal economy plays an important role in a game's dynamic behavior and gameplay. Just as feedback plays an important role in any dynamic system (Fromm 2005), it plays an equally important role in a game's internal economy. The idea of applying the concept of feedback to games is not new. During his 1999 lecture at The Game Developers Conference Marc LeBlanc introduced feedback loops to the game design world (1999). Since then, feedback loops have been discussed by a number of influential designers, including (Salen & Zimmerman 2004, Adams & Rollings 2007, Fullerton 2008). A classic example of feedback in games can be found in Monopoly where the money spent to buy property is returned with a profit because more property will generate more income. This feedback loop can be easily read from the Machinations diagram of Monopoly: it is formed by the closed circuit of Resource Connections and State Connections between the money and property pools. More specifically, for feedback to exist, a close circuit of connections is required that consists of at least one state connection that is not an activator. A closed circuit of resource connections can only create a loop of resources. To change the rate of the flow, at least one Label modifier, Trigger or Activator that changes or triggers the production or consumption of the resources must be part of the loop. Note that for this reason a closed circuit of resource connections that includes a converter or trader also constitutes a feedback loop, as their equivalent constructions do include a state connection.
Positive and Negative Feedback
As is the case in classic control theory, Marc LeBlanc distinguishes between two types of feedback: positive and negative feedback. Positive feedback strengthens itself and destabilizes a system. Positive feedback occurs when an effect fuels itself. In the Monopoly example above, the feedback is positive because investing money will generate more money. Positive feedback amplifies small differences between players: a player that has a lucky break early in the game will find this luck amplified over time: a player that by chance gets the option to get more money or good property early in a game of Monopoly is very likely to win. Positive feedback can be applied to 'positive' game effects but also to 'negative' game effects, as is the case with loosing pieces in Chess, which increases the chances of loosing more pieces, and which will eventually make you lose. LeBlanc suggests that positive feedback drives the game to a conclusion and magnifies early successes (Salen & Zimmerman 2004; 224-225).
Negative feedback is the opposite of positive feedback. It stabilizes a game by diminishing differences between players, by applying a penalty to the player who has done something that takes him closer to his goal and winning the game, or by giving advantages to the trailing players. Many racing games use negative feedback to keep a race close, either by giving trailing players more advantages or by hindering leading players. This effect is often described as 'rubber-banding'. It can be implemented by blatantly giving trailing players better acceleration and more grip, or more subtely as is the case in Super Mario Kart by having the most effective weapons in the game affect cars in front of the player that uses them. LeBlanc points out that in most multiplayer games that allow direct interaction, some sort of negative feedback is already in place: rational players will target the leader more than any other player. As one might expect, negative feedback can prolong a game and magnifies late successes.
Control theory, in almost all cases, strives for negative feedback while avoiding positive feedback, as it aims to create stable systems. A large part of control theory concerns itself with determining and optimizing the stability of the system. For games the situation is, of course, very different. Positive feedback loops are much more frequent in games because, in general, players do not want to play a game that is stable and drags on forever. Negative feedback does have an application within games: most games with only positive feedback will seem too random to many players as they will be unable to catch the player who took an early lead; negative feedback is often used to balance out early successes.
Multiple Feedback Loops
Most games need multiple feedback loops to display truly interesting emergent behavior. The game Risk is an excellent illustration of this as in this game four feedback loops interact.
The core feedback loop in Risk involves the resources armies and territories. The figure below depicts this core feedback loop. The label '+1/3' of the Label Modifier that sets the output flow rate of the source 'build' indicates that the output of the source is improved by one for every three territories the player has.
The second feedback loop in Risk is formed by the 'cards' that are gained from a successful attack (see diagram below). Only one card can be gained every turn, thus the flow of cards passes through a Limiter Gate first. Collecting a set of three cards can be used to generate new armies. Not every set generates armies, and some sets generate more than others. In the figure below these effects are indicated by the random symbol labeling the output of the converter that converts cards into armies.
The third feedback loop is activated when a player manages to capture a continent, which will give the player bonus armies every turn (see figure below). In Risk predefined groups of territories form continents as indicated by the design of the game board. In the diagram this level of detail is not possible, instead the construction is represented as a pool connected to another pool with a Node Modifier. In this particular case, seven territories will count as one continent which will in turn activate the bonus source.
Finally, the fourth feedback loop is activated by the loss of territories due to the actions of other players (see figure below). Which player is going to attack which other player is dependent on many factors, including those players strategies and preferences. Sometimes it is opportune to prey on weaker players in order to gain territories or cards, sometimes it is important to oppose stronger players to keep them from winning. In the diagram this is indicated by the multiplayer dynamic label (an icon depicting two pawns) affecting the resource flow to the drain on the right. The number of continents a player captured has a strong influence on this. As in general, players do recognize that other players that have a continent have a big advantage and will usually act against that more vigor. The player might also lose armies to actions of other players. I have chosen to omit them from the diagram to avoid too much clutter. It should not affect the argument too much. The important thing is, that in Risk there is some form of friction caused by other players, and the strength of this friction increases when the player has captured continents. This type of friction is a good example of the negative feedback that can almost always be found in multiplayer games where players can act against each other as pointed out by LeBlanc (see above).
The first three feedback loops in Risk all are positive: more territories or cards will lead to more armies which will lead to more territories and cards. Yet they are not the same. The feedback of cards is much slower that the feedback of territories as a player can get only one card each turn, but at the same time the feedback of the cards is also much stronger. Feedback from capturing continents operates faster and even stronger. These properties are important characteristics of the feedback loops that have a big impact on the dynamics of the game. Players are more willing to risk an attack when it is likely that the next card they will get completes a valuable set: it does not improve their chances of winning a battle but it will increase the reward if they do. Likewise the chance of capturing a continent can inspire a player to take more risk than the player should. In Risk the player's risks and rewards constantly shift, making the ability to understand these dynamics and to read the game a decisive skill in this game. These three positive feedback loops play an important role but simply classifying them as positive does not do justice to the subtlety of the mechanics.