Truthfulness

Truthfulness is one of Fallout GURPS disadvantages. See: Fallout GURPS.

Overview
"You hate to tell a lie - or you are just very bad at it. To keep silent about an uncomfortable truth (lying by omission), you must make your Will roll. To actually tell a falsehood, you must make your Will roll at a -5 penalty! A failed roll means you blurt out the truth, or stumble so much that your lie is obvious"

- Steve Jackson Games GURPS sources

In Fallout
With the abandonment of GURPS, Truthfulness was removed from the game