Trait

In the SPECIAL character system, traits are optional character modifications that affect the player character. Fallout 3 and Fallout: Brotherhood of Steel are the only games where there are no traits.

Some trait were rolled into perks in Fallout 3 and they are to make a return in Fallout: New Vegas.

Effect
Traits modify different aspects of gameplay, including (but not limited to): Primary statistics, derived statistics and skills. Traits always have both positive and negative results. 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.