Hidden trait

Hidden traits are traits in The Sims 3 and The Sims 4 that cannot be seen in-game except when a Sim performs a related action autonomously or an unlocked interaction is available.

Hidden traits become available to Sims in specific careers, life states, or in some cases the children of a Sim with the hidden trait.

Standard hidden traits
These traits are hidden but otherwise function similar to the standard traits. These hidden traits are often found in NPCs and Sims who reach certain career levels. Sims with these traits have a 50% chance to pass them onto their offspring.

Technical hidden traits
These traits are assigned to Sims by the game for technical purposes and are not supposed to represent proper traits. Many of these hidden traits do not feature icons.

Trivia

 * If a Sim in a house has already bought the Fireproof Homestead lifetime reward, a pyromaniac Sim will not be able to set a fire on that house.
 * Despite the fact that pyromaniac's fire does not harm anything, it will still trigger the fire alarms if there are any nearby, summoning the firefighter and leading to a fine.
 * If a Sim has the good trait and the secret private eye trait, the Sim won't have the option to beat up other Sims for information.
 * The ability to sneak with the secret private eye trait is only available for young adults and older.
 * With the secret can apprehend burglar trait, if the Sim has the coward or good trait, they are unwilling to fight burglars.
 * Even though a Sim has the immune to fire hidden trait, they can still be caught on fire from inventing, making metal sculpture, taking an experimental treatment from the Science career, or from a fire trap. They will get the "Singed" moodlet, but they won't die if they catch fire again.
 * Sims can get the unicorn trait by using a 3rd party mod.
 * Using 3rd party mods or cheats to simply add the hidden traits for various occults does not properly apply occult status and will cause problems, unless the trait is "PlantSim".
 * If using a mod and selecting the Rocker trait in Create A Sim, an animation will be played, unlike the other hidden traits. However, there is no description.

The Sims 4
The Sims 4 features some hidden traits that are never visible. Many of them are used only for technical purposes, such as marking visited secret lots, completed first time actions, or to provide functionality for NPCs. The following hidden traits function similarly to regular traits, but are simply not visible.

Life state traits
Occult Sims will have these traits, though they can only be viewed when editing an occult in CAS and setting club membership requirements. There are no in-game descriptions.