Trait

In the SPECIAL character system, traits are optional character modifications that affect the player character. Traits have only been absent in three games to date: Fallout 3, Fallout: Brotherhood of Steel and Fallout 4.

Some traits—such as Bloody Mess and Finesse—were converted into perks for Fallout 3. Traits make a return in Fallout: New Vegas, their only appearance in a first-person Fallout game.

Effect
Traits modify different aspects of gameplay, such as primary statistics, derived statistics and skills. Traits have positive and negative results, unlike perks in earlier Fallout games (with the lack of traits in Fallout 3, some perks do have negative effects). In games where traits exist, one can choose up to two traits when designing a character (or none at all). Choosing the Mutate! perk lets the player change one of his traits. Note that if you chose no trait at character creation, you cannot fill the empty slot with this perk.

Race
In Fallout Tactics, where characters of different races can be created, traits also have different racial requirements. In Lionheart, a fantasy game which uses the SPECIAL system, a different approach to racial traits was chosen. For each race one is free to choose (or not) any of the regular list of traits, but if the character belongs to one of the "tainted" races (i.e. everyone except pureblood humans), the player has to choose one of the list of racial traits available to their race. These traits are the only thing that distinguishes the races from humans and often had no penalty other than marking the character as tainted.

Old World Blues traits
Traits added by the Fallout: New Vegas add-on Old World Blues.
 * Claustrophobia
 * Early Bird
 * Hoarder
 * Hot Blooded
 * Logan's Loophole
 * Skilled

Racial traits
Racial traits only appear in Fallout Tactics.
 * Beta Software
 * EMP Shielding
 * Domesticated
 * Fear the Reaper
 * Glowing One
 * Ham Fisted
 * Rabid
 * Targeting Computer
 * Tech Wizard
 * Tight Nuts
 * Vat Skin