The following is based on pre-release information for the canceled Fallout Online game by Interplay Entertainment (Codenamed Project V13) and is not considered canon.
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.
One can choose up to two traits when designing a character (or none at all). Similar to Fallout Tactics, traits in Project V13 would have had different racial requirements.
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