Hidden trait

Hidden traits are traits in The Sims 3 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 to babies with an NPC for a parent. The child might then get the appropriate hidden trait.

Burglar

The Burglar hidden trait can be found in children of Sims in the Criminal career.
Burglars must perfect their sneaking skills to properly liberate imprisoned items from the homes of neighbors. Burglars like to keep the skill in the family, so their children are able to sneak out like a pro at birth.

Abilities

Can "Sneak" instead of walking or running.

Can Apprehend Burglar

The Can Apprehend Burglar hidden trait can be found in NPC Police Officers' children and Sims working in the Law Enforcement career.
Police officers spend years learning how to best the most clever burglars. Luckily for their children, genetics helps them stop burglars without years of tedious training.

Abilities

Can fight any burglars on home lot.

Can Salute

The Can Salute hidden trait can be found in children of Sims in the Military career or Sims who work in certain level of the Military career. Sims who graduated from Fort Starch Military School also receive this ability.
Children of high-ranking military officials know enough about the military's rules and regulations to snap off a smart salute when they want.

Abilities

Can "Salute" other Sims.

Immune to Fire

The Immune to Fire hidden trait can be found in children of NPC Firefighters, Sims in the Firefighter profession, Bonehilda, and Genies.
Fire Fighters are able to survive countless fires; not least because of their natural immunity to fire. It's a trait that they pass on to their children.

Abilities

A Sim cannot be killed by fire.

Makes No Messes

The Makes No Messes hidden trait can be found in maids' children.
The descendants of maids have cleanliness imprinted on their genetic code so they never leave anything dirty.

Abilities

Never causes messes, similar to an attribute of the Neat trait.

Pizza Appreciator

The Pizza Appreciator hidden trait can be found in NPC pizza deliverers' children.
Pizza delivery folks develop a love of pizza that's deeper than the thickest crust. Their children share a similar fanaticism for pizza, which makes every bite of hot pizza amazing.

Abilities

Eating pizza grants a Sim the Amazing Meal moodlet.

Pyromaniac

The Pyromaniac hidden trait can be found in children of NPC firefighters.
You can only get this trait in a Sim who is the offspring of a firefighter Sim. The trait allows them to set fire to objects of their choice, and they get positive moodlets for doing so!

Abilities

Can transmogrify any fruit and turn it into a flame fruit.
Can set things on fire for a short time without destroying anything.
A Sim with the Pyromaniac trait can survive until three hours when on fire, instead of the regular one hour. Thus their moodlet will be positive.
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 the 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.

Rocker

The Rocker hidden trait can be found in children of Sims in the Music career.
Sims with this trait will get a natural feeling of rock stardom.

Asian Culture

The Asian Culture hidden trait can be found in children of Sims from Shang Simla.

Abilities

Chinese-themed idle animations are triggered on Sims.
The Sim uses chopsticks when eating.
The Sim sings Chinese songs autonomously.

Egyptian Culture

The Egyptian Culture hidden trait can be found in children of Sims from Al Simhara.

Abilities

Sings Middle Eastern songs autonomously.
Egyptian-themed idle animations.

French Culture

The French Culture hidden trait can be found in children of Sims from Champs Les Sims

Abilities

The Sim greets other Sims with a dual-cheek kiss (known in real-life France as La bise.)
The Sim sings French songs autonomously.
The Sim gets a discount when visiting France.

Mummy

The Mummyhidden trait can be found in Sims who are mummies.

Abilities

Makes a Sim behave like a Mummy.

PrivateEye

The PrivateEye trait is a hidden trait added with Ambitions. It is gained by reaching level 7 in the Investigator profession. This trait has no icon.

Abilities

Sims have the ability to sneak.
A Sim can bribe or beat up to convince another sim while completing an opportunity, even if they don't work in the Investigator profession anymore.
This hidden trait is passed on to their children.

SimBot

The SimBothidden trait can be found in Sims who are SimBots.

Abilities

Short Out upon entering water.
Able to destroy various household objects.
Eats scrap metal instead of food.
Doesn't have a hygiene need.
Able to get jobs, and have romantic relationships.
SimBots snore binary code (0's and 1's) instead of Z's.
Occasionally, when an electrical object breaks down, SimBots will approach it and mourn.
Can talk to the microwave.

Vampire

The Vampirehidden trait can be found in Sims who are vampires.

Abilities

Makes Sim behave like a Vampire.
Sims with this trait can "Show Off Teeth".
Vampires snore V's instead of Z's.

Imaginary Friend

The Imaginary Friend hidden trait can be found in Imaginary Friends or in any Sim who was once an Imaginary Friend. It has no icon.

Abilities

An Imaginary Friend can switch from "doll mode" to "Sim mode" and vice versa if they have already drunk the Imaginary Friend Metamorphium potion.
An Imaginary Friend that has not drunk the Imaginary Friend Metamorphium potion can only be seen by the owner of the doll.
This hidden trait is passed on to their children.

Unicorn

The Unicorn hidden trait can be found in Unicorns. It has no game icon.

Abilities

Gives unicorn special abilities.
Makes horses behave like unicorns.
Runs faster than normal horses.
This trait may be passed on to a horse's foal.

Genie

The Genie hidden trait can be found in Sims who are Genies. It has no game icon.

Abilities

Gives genie special abilities.
When running, a Genie will be floating.
This hidden trait is passed on to their children.

World Peace

The World Peace hidden trait can be found in Sims who have wished for World peace from a Genie. This trait is passed on to the Sims'children. It has no game icon.

Abilities

Sim can talk about "World Peace" with another Sim.

World Misery

The World Misery hidden trait can be found in Sims who have the Evil trait and wished for World Misery from a Genie. The trait is passed on to the Sims'children. It has no game icon.

Abilities

Sim can talk about "World Misery" with another Sim.

Ghost

The Ghost hidden trait is found in ghosts after installing the Supernatural expansion pack.

Witch

The Witch hidden trait is found in witches.

Abilities

Unlocks the magical interactions. However, this trait alone is not enough to unlock the Spell, Curse, or Charm interactions. The trait does unlock these interactions no matter what life state (if using a mod) the Sim is in.
1. Conjure Apple
2. Unlocks "Whiz on Magic," "Brag about Broom Riding," and "Enthuse about Magic" interactions.

Lycanthropy Human

The Lycanthropy Humanhidden trait is one of two hidden traits found in werewolves. Apparently, this trait appears in Sims in their human form, and it unlocks several interactions, but not werewolf abilities.

Abilities

Make Sim behave like a Werewolf.
Sims can "Joke about Shedding" and "Show Off Teeth".
Sims with this hidden trait cannot ask for the werewolf curse from other werewolves.
This trait does not enable Sims to transform into werewolf form.

Lycanthropy Werewolf

The Lycanthropy Werewolfhidden trait is one of two hidden traits found in werewolves. This trait is presumed to appear in Sims in their werewolf form. However, this trait does not unlock the abilities that appears on Lycanthropy Human trait.

Abilities

This trait does not enable Sims to transform into werewolf form.
Interactions from the other trait are not unlocked. On the other hand, Sims can ask for the werewolf curse even though they possess this trait.

Fairy

The Fairy hidden trait can be found in Sims who are fairies.

Abilities

Make Sim behave like Fairy.

Trivia

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.
There are four more hidden traits which have no effect on Sims. These are labeled "DNT", and come with The Sims 3: Showtime. These hidden traits cannot be obtained unless a third party mod is used.
With Supernatural installed, the Genie, the Spellcaster, and the Fairy hidden traits do not unlock all the interactions and powers available to the respective occults. One must be of the occult to have full access to the powers.
The Ghost hidden trait is a one that is introduced in Supernatural. Even though ghosts have-existed since the base game, the trait is featured in this expansion pack. It's unknown what differs it from ghosts without this trait, or whether existing ghosts are automatically given this trait. It may be related to the haunting curse ghosts.