An immersive sim (simulation) is a video game genre that emphasizes player choice. Its core, defining trait is the use of simulated systems that respond to a variety of player actions which, combined with a comparatively broad array of player abilities, allow the game to support varied and creative solutions to problems, as well as emergent gameplay beyond what has been explicitly designed by the developer.
Immersive sims by definition allow for multiple approaches, and typically incorporate elements of multiple genres, including role-playing games, stealth, first-person shooters and platform games. Although they typically have smaller worlds than open world games, they also generally allow for open-ended gameplay, allowing the player to progress in any order and pursue side missions alongside any main story missions. Immersive sims are generally compared to Thief and Thief 2, System Shock and System Shock 2, as well as the first Deus Ex, which are considered to be the earliest and most critically acclaimed titles in the genre.
The term "immersive sim" may also be used to describe the game design philosophy behind the immersive sim genre, which uses interacting, reactive and consistent game systems to create emergent gameplay and a sense of player agency.
Immersive sims typically task the player to make their way through levels and complete missions, but do not enforce the means by which the player does this. A common example would be where the player-character must get past a guard. The choice of how to do this would be up to the player: they may attempt to sneak around; use parkour or other similar abilities, aided with some equipment, to slip around him; find small passageways that allow them to get around the guard; create a distraction that draws the guard away from their post; convince or bribe the guard to ignore them; or simply attack and kill or disable the guard directly. The choices may be limited by the player-character's abilities and current inventory, and there may be consequences of the player's choice. For example, killing or disabling the guard could leave the guard's body to be found later, raising the alert level of other guards.
Warren Spector said that immersive sims create the feeling that "you are there, nothing stands between you and belief that you're in an alternate world". Many of the key developers of immersive sims compare them to tabletop Dungeons & Dragons games hosted by a good gamemaster, or to Live action role-playing games, in that there are a set of rule systems to keep it a game, but the game will react to the players' actions rather than force the player to conform to a specific action. Spector is credited with the term "immersive sim" in a post-mortem he wrote on the development of Deus Ex in 2000, although Spector himself attributes it to his Looking Glass colleague Doug Church instead.
Mark Brown of the YouTube series Game Maker's Toolkit identified that a key differentiating feature of immersive sims is that they do not readily use scripted or fixed events. Instead, they use a consistent series of rules and systems throughout the game. These consistent systems then can be exploited by the player to complete objectives in unique and unpredictable ways, with the game reacting to the player's decisions. Brown uses the example of being able to fire rope arrows (to climb on) at any surface in the original Thief: The Dark Project (1998), while the 2014 Thief game limited what locations these could be used, removing the immersive sim elements. Rick Lane of PC Gamer noted that while earlier games in The Elder Scrolls series were not immersive sims, The Elder Scrolls IV: Oblivion (2006) transitioned the series to an immersive sim.
Warren Spector considered Ultima VI: The False Prophet (1990) the first game to have an the immersive sim mentality as while played from an top-down view, it relied less on events and planned-out puzzles, and instead provided the rulesets and systems through its living world to allow players to craft their own solutions to situations. Spector described one playtesting example from Ultima VI that he considered the genesis of the immersive sim genre, in which a playtester lacked a magical spell needed by his party to pass by a closed gate, and instead used a pet mouse character to sneak through small spaces and access the necessary controls to open the gate, something none of the developers had anticipated.
Ultima Underworld: The Stygian Abyss (1992) is considered the first game to demonstrate the necessary elements of an immersive sim according to Spector and others. It build up on Ultima VI's gameplay and added in the first-person perspective, predating Wolfenstein 3D, the game that launched first-person shooters, by a few months. The first-person view helped to cement the impression that the player was part of the game's world that they had full control of, and completing the impression of immersion. Spector recalled that he had thought to himself "Do you not realize that the entire world just changed?" on seeing the initial demo for Ultima Underworld. Other early examples include System Shock (1994) and its sequel System Shock 2 (1999), Thief: The Dark Project (1998) and its sequel Thief II (2000), Deus Ex (2000), and Arx Fatalis (2002). However, at the same time, more action-oriented games with strong narrative elements that followed from Wolfenstein 3D, like Doom (1993) and Half-Life (1998), drew larger commercial sales, making it difficult to gain publisher interest.
Around 2006-2008, several games emerged that revitalized interest in the immersive sim, including The Elder Scrolls IV: Oblivion (2006), BioShock (2007), S.T.A.L.K.E.R.: Shadow of Chernobyl (2007), The Witcher (2008) and Fallout 3 (2008). Spurred from these successful titles, there have been new titles in the Deus Ex series, including Deus Ex: Human Revolution (2011), as well as a planned System Shock 3 and Underworld Ascendant releases. New properties, including Dishonored (2012) and Prey (2017) were developed acknowledging the design principles of immersive sims.
While the immersive sim genre is well-received critically, their sales have been varying. The original Deus Ex sold more than 500,000 units (at the time, a respectable number), but its immediate sequels, Deus Ex: Invisible War, was considered a commercial failure. More recently, while Deus Ex: Human Revolution sold more than 2.1 million copies within a month of its release, its sequel Deus Ex: Mankind Divided had not yet cleared one million in sales a year after its release. Dishonored 2 also did not see similar sales uptake as the original Dishonored. While Jody Macgregor of PC Gamer noted that there are other factors contributing towards lower sales, including other competing games, and changes in a sequel's marketing and approach, they noted that immersive sims require more commitment from the player to invest and learn the game's complex interacting systems in contrast to other types of games built around simpler mechanics, making immersive sims harder to sell. Jordan Thomas, a developer on Thief and the BioShock games, also said that immersive sims very difficult to be build by groups other than large teams due to the complexity of such games, making contributions from indie games unlikely. Arkane's Harvey Smith believed that while these recent sales trends for immersive sims are disappointing, there will always be a market for them, but there will be a need to balance the cost of development to lower sales numbers. Smith attributes the lower sales of more recent games to the general trend of players favoring fast-paced action games with strong multiplayer components, with publishers being wary of games without such elements. Immersive sims by nature tend to be single-player experience requiring thought-out approaches, but Smith believes that the genre will adapt to these player preferences in the future, particularly from indie game development.
A small number of studios and developers have been associated with the immersive sim genre, creating a lineage in its development principally from Looking Glass Studios and its projects. Ultima Underworld was created by Paul Neurath and Doug Church under their studio Blue Sky Productions, which they took to Origin Systems for publication. Warren Spector of Origin, who had worked on Ultima VI, eagerly worked closely with Blue Sky to finalize the game, transitioning Blue Sky into Looking Glass, which created the System Shock and Thief games. Spector later joined Ion Storm and founded its Austin, Texas, studio, where they developed the Deus Ex series. Looking Glass Studios eventually closed down, but developers from it launched their own studios; notably, Ken Levine, who had helped with Thief and System Shock 2, left to form Irrational Games, creating the BioShock series.
Separately, Raphaël Colantonio had been part of the quality assurance team supporting Origin Systems for Electronic Arts (EA) that were publishing games like System Shock in Europe. Colantonio left EA and eventually founded Arkane Studios, desiring to make an immersive sim sequel to the Ultima Underworld series. EA denied them the use of the intellectual property, and instead Arkane produced Arx Fatalis. Later, Colantonio brought on Harvey Smith, a quality assurance tester for Origin for System Shock and one of the lead developers under Ion Storm for Deus Ex. They subsequently designed the Dishonored series as well as building the new Prey atop similar immersive sim fundamentals.  More recently, Neurath founded a new studio OtherSide Entertainment, obtaining the rights for an Underworld sequel from EA, and for the System Shock property through Night Dive Studios whom had acquired the rights from EA. Neurath brought on Spector to help create both sequels.
In immersive sims that include numeric codes, several of these games use the numerals "451" as part of the first code that the player encounters. The origin comes from both System Shock games which use it as part of the first door codes seen in the game, and itself a reference to Fahrenheit 451. Since then, its reuse in immersive sims is described as a "kind of a signature that developers use to align themselves with Looking Glass", according to former Looking Glass developer Tim Stellmach. The name "Looking Glass" is also played upon by developers of immersive sims to reflect on the importance of Looking Glass Studios to the genre. OtherSide Entertainment took its name as a play on "Looking Glass". Prey includes the "Looking Glass" computer system that the player encounters frequently on the station.
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