Reaction determines the character's initial attitude, which can be one of three results:
- 0 or less: Bad reaction.
- 1 to 50: Neutral reaction.
- 51 and above: Positive reaction.
The calculation is made when the player first talks to a character. The factors involved are:
- Charisma: +5 per point
- Presence: +10 per level
- Karma: Apply its current level
- Childkiller: -10
- Champion: +20
- Berserker: -20
The mechanic is functionally irrelevant. The vast majority of characters do not react differently based on their reaction. Even if they did, the system is further crippled by the fact that no characters are flagged as evil in Fallout, meaning the perk Cult of Personality is functionally useless. Fallout 2 correctly flags characters as good and evil, but the reaction system is largely ignored and Cult of Personality is broken.