Incidentally, Physicist Niels Bohr came up with a theory: the one who draws second moves faster because he reacts without thinking. Psychologists at Birmingham University found Bohr was right, at least to a point. Volunteers averaged 10% faster when they drew second over first. Andrew Welchman's study found that while a gunslinger moved faster when they drew second, the difference was on average only 21 milliseconds - too slow to beat someone who had already pulled a gun.
It appears to be accuracy, not who drew 1st that decides the victory.