When our interest in sex starts to wane, exposure to a new or novel partner has a way of bringing it back. This phenomenon--formally dubbed the Coolidge Effect--got its name from a popular anecdote about a visit that U.S. President Calvin Coolidge and his wife supposedly made to a chicken farm. The story goes something like this:
“Mrs. Coolidge, observing the vigor with which one particularly prominent rooster covered hen after hen, asked the guide to make certain that the President took note of the rooster’s behavior. When President Coolidge got to the hen yard, the rooster was pointed out and his exploits recounted by the guide, who added that Mrs. Coolidge had requested that the President be made aware of the rooster’s prowess. The president reflected for a moment and replied, ‘Tell Mrs. Coolidge that there is more than one hen.’” 
The Coolidge Effect has been documented in several animal species. For instance, research has found that when a male rat is placed inside a cage with several female rats that are in heat, he will mate with all of them until he appears exhausted. However, if a new female is then introduced to the cage, males often experience an immediately renewed interest in sex and begin mating with her .
The Coolidge Effect has been documented in humans as well. For instance, in one study, male participants were either exposed to constant or varied sexual stimuli while their level of sexual arousal was measured by a device that records changes in penile circumference . The men who were repeatedly shown the same stimuli showed less arousal over time (in other words, they demonstrated habituation); by contrast, men who were exposed to varied stimuli maintained higher levels of arousal.
Another study found that, after watching porn clips featuring the same actress over a period of several days, exposure to porn featuring a new actress was linked not only to faster ejaculation, but also the release of more active sperm . This suggest that the Coolidge Effect may have an evolutionarily explanation behind it in that it might potentially increase men's odds of reproductive success with new partners.
The Coolidge Effect has also been documented in females, although the pattern tends to be somewhat less pronounced. For instance, research on female hamsters has found that after mating with one male hamster until exhaustion, they demonstrate renewed interest in sex when a new male is introduced to the cage . Also, research on women has found that, just like men, they show some degree of habituation in response to repeated presentations of the same erotic stimulus . What this tells us is that the Coolidge Effect isn’t a uniquely male phenomenon by any stretch of the imagination.
"Declining sexual interest in a long-term partner and being excited by variety is probably to be expected, rather than a sign that there's something wrong with you or your relationship."
As you might imagine, the Coolidge Effect has important implications for our romantic relationships. In particular, it suggests that declining sexual interest in a long-term partner and being excited by variety is probably to be expected, rather than a sign that there's something wrong with you or your relationship.
So what can a couple do to combat this potential decrease in sexual interest?
Some might decide to have a consensually nonmonogamous relationship, in which they explicitly permit some degree of outside sexual involvement. This can take many different forms (e.g., having an open relationship, swinging, the occasional threesome). Of course, consensual nonmonogamy isn’t right for everyone. Different types of relationships work better for different people, in part, due to differences in personality.
For those who want help fending off the Coolidge Effect while maintaining monogamy, one way they can accomplish this is by incorporating more novelty into their relationship and sex life. Novelty in all forms (not just new partners) can breed sexual excitement. As some evidence of this, research has found that the long-term couples who report having the most intense feelings for each other are those who engage in the most new and exciting activities together . In other words, you can potentially stimulate that same level of sexual excitement that you might receive from a new partner by bringing more novelty into your relationship in other ways.
All of this is to say that trying new things and sharing new experiences (sexual and non-sexual) is one of the keys to keeping passion alive.
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 Hatfield, E., & Walster, G. W. (1978). A new look at love. Lanham, MD: University Press of America.
 Wilson, J. R., Kuehn, R. E., & Beach, F. A. (1963). Modification in the sexual behavior of male rats produced by changing the stimulus female. Journal of Comparative and Physiological Psychology, 56, 636.
 O'Donohue, W. T., & Geer, J. H. (1985). The habituation of sexual arousal. Archives of Sexual Behavior, 14, 233-246.
Joseph, P. N., Sharma, R. K., Agarwal, A., & Sirot, L. K. (2015). Men ejaculate larger volumes of semen, more motile sperm, and more quickly when exposed to images of novel women. Evolutionary Psychological Science, 1(4), 195-200.
 Lester, G. L., & Gorzalka, B. B. (1988). Effect of novel and familiar mating partners on the duration of sexual receptivity in the female hamster. Behavioral and Neural Biology, 49, 398-405.
 Kelley, K., & Musialowski, D. (1986). Repeated exposure to sexually explicit stimuli: Novelty, sex, and sexual attitudes. Archives of Sexual Behavior, 15, 487-498.
 O’Leary, K. D., Acevedo, B. P., Aron, A., Huddy, L., & Mashek, D. (2012). Is long-term love more than a rare phenomenon? If so, what are its correlates? Social Psychological and Personality Science, 3, 241-249.
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