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The Coolidge effect is a biological phenomenon seen in animals, whereby males exhibit renewed sexual interest whenever a new female of reproductive availability is introduced, even after sex with prior but still available sexual partners.[1][2][3][4] To a lesser extent, the effect is also seen among females with regard to their mates.[3]

The Coolidge effect can be attributed to an increase in sexual responsiveness, and a shortening of the sexual refractory period.[5] The evolutionary benefit to this phenomenon is that a male can fertilize multiple females.[6] The male may be reinvigorated repeatedly for successful insemination of multiple females.[7] This type of mating system can be referred to as polygyny, where one male has multiple female mates, but each female mates with only one or a few males.[5] The Coolidge effect has been demonstrated to occur in humans across cultures and in both sexes.[8]