Why Left 4 Dead Works

By Paul Goodman,Adams Greenwood-Ericksen

[What makes the Left 4 Dead series' gameplay design work so well for players? This comprehensive design analysis analysis from Full Sail grad student Paul Goodman and course director Adams Greenwood-Ericksen takes developer commentary and matches it against the SRK Framework, a tool for categorizing cognitive behavior in interactive media.]

Mechanics are the core building blocks of any game. They are the primary method by which players are able to operate in a game environment to overcome the challenges and obstacles that a game's developers have placed down. Yet in their design a crucial factor can be overlooked with regards to how game mechanics are made or adjusted through the development process: understanding the ways in which players think and learn about the function of game mechanics is key to determining their effectiveness.

If a mechanic is too complex for players to understand, the result may be player frustration, whereas if a mechanic is too simple, it can quickly lead to player boredom and repetitive gameplay. By using established methods of assessing and modeling cognitive behavior, game designers might gain better insight into the likely thought processes of players, enabling them to improve the quality of mechanics in game.

One such method, the Skills, Rules, and Knowledge Framework (SRK), which was first presented by Jens Rasmussen in 1983, focuses on categorizing different types of cognitive behavior across three different levels.

A strength of the SRK framework is that it is broadly applicable to virtually all interactive activities. Therefore, in the context of electronic gaming, players of a particular video game will engage its mechanics using cognitive behaviors that can be assessed and classified by designers using the SRK Framework.

Therefore, game designers can use this tool to identify how players engage a game mechanic during gameplay. Through reviewing how players move from a knowledge based behavior to rule based or skill based behaviors (or vice versa), designers can assess the effectiveness of a game mechanic and make adjustments as necessary, such as simplifying or adding complexity to the mechanic itself.

An example of a game that was developed with clear emphasis on what and how players think can be found in Valve's Left 4 Dead, released in 2008, and its sequel Left 4 Dead 2, released a year later in 2009. An action/survival first person shooter, L4D puts the players in control of four survivors battling their way to safety during a zombie apocalypse.

Hordes of "Infected" bar their path at every turn throughout the four campaigns, and working together as a team is often the only way players are able to achieve success, especially as many of the challenges posed by game mechanics and obstacles players face require more than one individual to complete.

Left 4 Dead

Early design of Left 4 Dead came about in 2004 after the release of Valve's classic tactical shooter Counter-Strike: Source. According to L4D team member Mike Booth, while experimenting with the AI for non-player bots the dev team found that the concept of "Small team of friends against hordes of clawing enemies" had a great deal of potential. Through more playtests and design discussion, the framework was laid down for Left 4 Dead core concept of a "Co-op vs. the horde game."

Players of L4D find themselves battling against the Infected, victims of a quickly and highly contagious plague that's ravaging the U.S. At their disposal, players start off with a pistol (with unlimited ammo) and a default melee attack for pushing enemies away; throughout each campaign players find more weapons ranging from pump-action shotguns to fully automatic assault rifles, as well as improvised items in the forms of Molotov cocktails and pipe bombs.

Common Infected are the most frequently encountered enemy type within the game, located almost everywhere in each campaign. They appear as normal people, albeit with glowing eyes and bleached skin. The average Infected is a fast runner, can move through most terrain and over most obstacles and assault players with punches and kicks. However, they die fairly easily with a few well placed shots to their chest, head, legs or arms (the horror movie standard of zombies dying only from head wounds having been thrown out).

They also are lacking any sense of self-preservation, and will run through and into hazards and obstacles, including fires and live explosive devices. Occasionally, common Infected attack in large groups (referred to by players as a "Horde") and try and overwhelm the players using their superior numbers. This can be a random occurrence or triggered by a scripted "crescendo" event, usually caused by players having to overcome an environmental obstacle such as a lowering a bridge or destroying a barricade.

The common Infected serves a variety of different purposes, the most obvious of which is to serve as the main enemy type opposing players, but also to provide experience with core gameplay dynamics such as the use of the weapons and items at the player's disposal.

The simplicity of the common Infected allow new players to L4D to quickly evolve during their normal progress through the game environment from knowledge-based behaviors involving research and experimentation intended to determine how common Infected act, to using rule or skill-based behaviors to quickly and efficiently overcome even large numbers of these basic enemies.

AI Director

Controlling the common Infected as well as other types of game mechanics such as the placement of weapons or health kits is the AI Director, a module that works behind the scenes of the game and may be considered the "intelligence" directing what the player experiences during gameplay. The AI director monitors the actions and conditions of players and uses that information to make changes to the game environment in real time.

According to L4D team member Gautam Babbar, game levels don't have pre-scripted placement of enemies: instead, the AI Director is comprised of a "system that tracks each Survivor's 'stress level' by watching for events like 'how much damage you are taking', 'how many zombies have you killed near you', and so on.

If a Survivor's stress gets too high, the system will step in and forcibly throttle back the zombie population system to make sure the team gets a break every now and then." In this way, the role of L4D's AI Director could be seen as determining if players are engaging in effective rule/skill/knowledge based behavior and raise or lower the difficulty or number of enemies accordingly.

Special Infected

However, through play-testing early versions of the game, the development team found that in order to expand and improve the gameplay experience they would need to include additional game mechanics to complement the Common Infected enemy type that populated the game world.

This lead to the Boss/Special Infected, which were developed, according to L4D team member Sean Keegan "directly from weaknesses observed during... daily playtesting." As there was only one enemy type present in the game, players exhibited a more-or-less skill based behavior of shooting everything that moved, which quickly became repetitive and tedious.

The five types of special Infected in L4D are horribly mutated creatures that are randomly spawned in the game environment by the AI Director to further impede a player's progress through the game environment. Each come with their own unique actions in game designed to fill a specific niche with abilities that the common Infected lack, such as disrupting a team's ability to protect one another.

For example, the Smoker, which uses a ranged attack to drag players towards its claws, was created to pose a threat to experienced players who stayed in tight groups when moving through the level. Team member Khanh Nguyen summarized the role of this dynamic as follows:

"Highly cooperative and experienced Survivor teams can become very good at staying together and watching all around them, to the point that it became difficult for the zombies to cause the kind of chaos and panic that makes the game exciting. The Smoker was created to forcibly pull these tight teams apart. Moving just one Survivor far out of position with the Smoker's tongue attack proved to be enough to break things up and bring the chaos back."

The Smoker's presence also encouraged teams to be aware of their proximity to each other in the game environment, as players who either lagged behind or charged ahead of their teammates became more vulnerable to the Smoker's attack and reduced their chances of a timely rescue.

The other special Infected have similar objectives in mind; the sweatshirt- wearing Hunter leaps great distances onto a target player, pinning it to the ground and tearing into them with its claws. Players that are too close to the hunter when it pounces onto its target can be knocked aside and temporarily stunned and only by killing it or using the default melee attack can players save a hunter's victim from disembowelment.

On harder difficulties or when under attack by other enemies, the few precious seconds lost while stunned can mean the difference between surviving the Hunter's assault or losing a team member; like the Smoker, players were encouraged by the Hunter to stay close to each other for protection, but not too close as to take collateral damage from his pounce.

Recognizing the cognitive behaviors of players allowed Valve to fine-tune and develop the special Infected to make them efficient, challenging and able to play off of each other's strengths and weaknesses. An example of this can be found in the development of the Boomer special Infected.

Initially, the Boomer first was an enemy that players wanted to avoid shooting at close range, as upon death he would detonate and cause heavy damage to nearby players. Its role of drawing down hordes of Infected onto players was held by another type of special Infected called The Screamer, a prototype enemy type created early in development whose role was discussed by team member Jaime Sue:

"The trick to the Screamer was that if he saw you, you had a moment to kill him before he ran away. Once the Screamer got away from the Survivors to a hiding place, he would emit a loud howling scream that would cause a huge mob of zombies to attack the Survivors. While there were several exciting moments of knowing you had to chase him down to shoot him before he screamed, dodging zombies all the way, ultimately it proved too confusing for the Survivors to discover how he worked, or even to reliably notice him in the crowd."

After observing playtests, developers noticed players had trouble engaging the Screamer game mechanic effectively, and cut it from the game while incorporating some aspects into the Boomer's abilities - the Screamer's 'scream' was converted into the Boomer's horde summoning vomit attack.

From the developer commentary, it can be derived that players were able to engage both the Screamer and the Boomer with similar knowledge-based behaviors during their first encounter with each special Infected; with no prior experience of either type playtesters would treat them like normal special Infected until they were able to develop processes to deal with each one's special ability.

However, it was easier for players to understand how the Boomer worked and develop rules and skills based behaviors to counter it; players would learn fairly quickly that it was best engaged at range and that being close to a Boomer when it was killed was not a good strategy. This level of understanding allowed players to handle the Boomer using rule or skills based behaviors.

The Screamer, on the other hand, blended in with the regular common Infected before running away from the player to begin its horde-drawing scream; players may have been able to engage this game mechanic using knowledge based behavior but were unable to understand or develop effective processes at the rule or skills based level in dealing with the Screamer, which therefore lead to player frustration.

With the revised Boomer, players were able to go from knowledge-based behaviors of first treating it as a common Infected while going through the learning process of recognizing its shape, how it sounds, and the unfortunate effects of being caught in either its vomit attack or shooting it point blank.

From these experiences, players were better equipped to develop their own rule-based behaviors on dealing with the Boomer in order to defend themselves and their teammates by engaging at range, avoiding its vomit attack, and using the default "push away" melee attack to stay out of its post-death blast radius.

More experience would lead some players to develop the automatic, triggered automatic processes included in Skill-based behaviors to efficiently engage the Boomer game mechanics.

Left 4 Dead 2

In developing Left 4 Dead 2, a sequel starring four new survivors working their way through a heavily Infected Louisiana, several changes were made to basic mechanics from the first game along with changes to environmental design.

Melee attacks now came with a cool down timer, forcing players who made liberal use of it in L4D to think tactically or endure a progressively longer wait between each swing. New weapons were also introduced, most notably hand-to-hand combat weapons such as fire axes, frying pans and katanas, while the players ability to use their unlimited-ammo pistols was removed in exchange for the melee weapon's close-range instant-kill attacks versus most common and special Infected.

Campaign-specific Uncommon Infected were introduced, namely to add some variety to the hordes of common enemy types and some minor additions to that aspect of game play as well; Hazmat suit-wearing Infected enemies encountered in the first campaign are immune to fire and Clown Infected during the "Dark Carnival" campaign draw small groups of Infected to them with their squeaking, oversized shoes.

Many game mechanics such as crescendo events and the special Infected enemy types carried over with few significant changes; experienced L4D players would already have a grasp of the established mechanics and thus be ready to face the repeat performances of the Boomer or the Hunter in the new campaigns.

The largest addition to the Infected rogues gallery were three more types of special Infected, introduced to counter player behaviors or actions that were commonly observed in experienced players following the release of the first game. Designers noticed that players, when faced with incoming hordes or dealing with a crescendo event, often would use enclosed spaces or corners to bottleneck attacking enemies.

The L4D version of the melee attack could be used to quickly defeat any special Infected that managed to break through or rescue teammates fairly easily. As team member Phil Co pointed out, "Left 4 Dead 1 had done such a good job of training people to stick together it was incredibly difficult to get them to separate." To encourage more creative game play, and to provide a new challenge to players, these new mechanics were introduced to help add more variety to L4D2's game dynamics.

Although the Smoker and the Hunter were already aimed at breaking up tightly packed teams, their effectiveness decreased when inside buildings or at environmental bottlenecks such as a hallway or door. Valve introduced the bulky Charger, a sort of miniature version of the Tank, which wields one massive arm to pick up and pummel a player into the ground to provide a counter to players' tactical use of such areas.

This special Infected, when attacking the player, first hurtles through the environment in a straight line at high speed, knocking anyone in its path aside until either impacting a solid object such as a wall or in some cases going straight off of a cliff or other level hazard. Often the first player to be caught in its way is dragged along for the ride, and when grabbed by the Charger's oversized fist, could only be rescued by killing it, as the Charger has immunity to a player's default melee attack.

Experienced L4D players would need to develop new rules and skill based behaviors regarding their environment and the safety of funneling enemies through doors, as the Charger introduced new risks to using those types of environmental bottlenecks. Chris Carollo noted that:

"In Left 4 Dead 1, long narrow hallways usually meant relative safety for the survivors. One player could cover the front and another could cover the rear, while players in the middle healed. With the addition of the Charger, however, the long narrow hallways in Left 4 Dead 2 turn out to be extremely dangerous."

Further adding to the dangers of standing ground or bunkering is the Spitter special Infected. The tall, gangly Spitter launches gobs of stomach acid at players from her jawless face which, on impact, form pools of burning goo at their feet, prompting a quick escape to avoid harm. Tom Leonard explains that this area of denial attack can "quickly force a group of survivors out of a tight spot, or drive a wedge between members of a group," as remaining in the pool results in increasing damage to the player.

Players in an enclosed space such as a room or closely grouped together (at this point, a common rules/skill based behavior versus other types of enemies) are forced to split up and/or move from safety in order to engage the Spitter to prevent her from repeating her attack. This would increase their vulnerability to attacks from common or other special Infected enemies. Like the Charger, the player is pushed to use knowledge based behavior to develop new strategies to deal with the Spitter and the new risks now associated with previously safe areas.

The last new special Infected, called the Jockey, serves as a 'wild card' to the other special Infected. The smallest enemy type in the game, the Jockey resembles a hunchbacked, scrawny man that leaps onto a player and essentially hijacks their controls.

Unless rescued by other players the Jockey will force players away from the safety of their team or location into nearby hazards (such as fire or Spitter acid) or into the range of other special Infected characters.

As a game mechanic, players would have to adjust their previously established rules/skills based behavior to include how to potentially deal with the randomness of a Jockey being included in any interaction with the special Infected or common Infected as game mechanics.

With a total of six special Infected enemy types in L4D2 with abilities to counter a wide variety of player behavior, experienced players of the first game would now have develop new behaviors to deal with the possible encounters or combination of encounters they would have during the course of the games campaigns.


In summary, the use of SRK Framework or similar methods of measuring player's cognitive behavior can assist game designers in identifying weak game mechanics, and help devise improvements or the creation of other complimentary game mechanics.

By recognizing that players were quickly able to learn the properties of the common Infected enemy type in L4D and engage them at rules or skill-based behavior, Valve was able to develop game mechanics in the form of the Special Infected to provide a greater challenge for players through introducing more risks to the easier learned types of skills or rules-based behavior versus Common Infected.

In addition, the special Infected helped reduce the repetitiveness of engaging the Common Infected game mechanic. By providing a means of assessing what constitutes a functional game dynamic at a cognitive level, the SRK framework can help game designers to interpret and apply lessons learned from playtesting and prototypes to improve game play and enhance the quality of the player experience.

In game development, the SRK Framework can be used during the design and testing of game mechanics can help reduce player frustration, as reviewing how players interact and engage a particular mechanic can identify their strengths and weaknesses.

Additionally, using SRK Framework in the cross-examination of a game mechanic's functions might also lead to the development of additional, complimentary mechanics to add more to game play and help reduce feelings of repetitiveness. As evidenced in L4D and L4D2, having a variety of different mechanics that shift players out of familiar skill-based behavior into knowledge-based introduces new challenges for players to overcome and therefore reduce repetitiveness.

Overall, this could lead to higher quality games with strong mechanics which maintain a good balance between challenging the player and retaining a simple and understandable dynamic that supports learning and improvement.

Measurement of player's cognitive behavior can assist game designers in identifying types of player behaviors for use in developing new game mechanics for a sequel or expansion. As the basis for their creation was primarily to counter player behaviors observed in L4D and introduce new challenges and experiences for players to overcome, Valve created the Jockey, Charger and Spitter for L4D2 to bring further variety to special Infected encounters in game and reduce monotony.

These new mechanics helped reduce veteran L4D players' feelings of repetitiveness in experiencing reused game mechanics from the first game as well as adding more content and mechanics for newer players to experience. Recognizing the skill-based and rule-based behaviors that players engage in while playing a game can allow Game designers to introduce new or adjust previously established mechanics either during the development process or in subsequent titles.

The use of such methods not only require veteran players to develop new behaviors to overcome the new challenges but also give newer players a wider variety of complex mechanics to engage, thus enhancing the quality of the gameplay experience for all players.

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