Mental health is an important game mechanic in Fahrenheit: Indigo Prophecy.
During the events in Fahrenheit's plot, the three protagonists—Lucas Kane, Carla Valenti and Tyler Miles—will face actions or events that will influence their mental health. This variation could have resulted from an arbitrary action by the player, or even be uncontrollable and unpredictable like resulted from NPCs or plot itself.
Mental health may increase or decrease and is represented on a bar and calculated in percentage points from 0 to 100. If the mental health hits 0%, a game over happens and the game can no longer be completed. The game explains this condition with various narrative expedients, for example Lucas Kane could end up in Bellevue Asylum or killing himself, while the agent Carla Valenti could have an emotional shock and resign as a police officer.
Each of the three characters has its own separate mental health, but of course it's enough that only one of these three reaches 0% to cause a game over.
Mental health is also expressed per literal definition, which changes based on how much the percentage goes up or down. For example a "100%" mental health indicates a state called "Neutral", while a "40%" mental health indicates a state defined "Depressed". On the mental health bar there is also an iconography of the character, a black shape, that graphically represents its mental state. Mental health score does not affect the aesthetic appearance of the character. It's possible to play with 5% mental health, and the character walks, runs or acts normally. Hallucinations or emotional delirium do not come into play: mental health is simply a "how much you're close to game over" counter.
Mental health percentage can change by a minimum of 5 or only by its multiplies, 10-20-30, which are the only ones operating in the game—except for example a "-60%" that occurs when Lucas Kane realizes he killed John Winston in The Murder chapter. For this reason there will never be a percentage mental health score like 6, 11 or 21 and so on, alike there will never happen changes based on 15 or 25 even if they are multiplies of 5.
Although it's a rare occurrence, and easier to be generated on purpose, it can happen that if the mental health is low—30% or less—and thus reduced to zero by mandatory plot events, impossible to skip, it can lead the game to a dead end where it's necessary to reload a saved game much back in the chapters.