In battle
Generation IV
When using a move, Rivalry raises the power of the move by 25% if the target and the Pokémon with this Ability have the same gender, but reduces base power by 25% if the target and the Pokémon with this Ability have opposite genders. If the Pokémon with this Ability or the target is genderless, the move's power is unaffected.
Unless the Pokémon is genderless, Rivalry powers up confusion damage.
Generation V onward
Rivalry no longer powers up confusion damage.
Outside of battle
Rivalry has no effect outside of battle.