The Origin of Emotions identifies the purpose, trigger and effect of each emotion. The emotion of revenge is an example. Revenge is a negative mental effect triggered when you conclude "X harmed me by breaking the rules". The negative effect continues until you conclude "I harmed X as much as X harmed me". Revenge evolved to coerce the victims of rule breaking to retaliate, whether it helps them or not. Because revenge coerces victims to retaliate, there are fewer rule breakers. The fewer rule breakers there are, the more efficient a group is. The Origin of Emotions is an attempt to nudge Evolutionary Psychology to its next logical step - the identification of each emotion by its unique purpose and trigger.