In biology and psychology, the Coolidge effect is a phenomenon seen in mammalian species whereby males (and to a lesser extent females) exhibit renewed sexual interest if introduced to new receptive sexual partners, even after refusing sex from prior but still available sexual partners. The evolutionary benefit to this phenomenon is to populations where one male fertilizes his entire harem. The male must be reinvigorated repeatedly for successful insemination in multiple females. 
Origin of the term
Behavioral endocrinologist Frank A. Beach first mentioned the term "Coolidge effect" in publication in 1955, crediting one of his students with suggesting the term at a psychology conference. He attributed the neologism to:
… an old joke about Calvin Coolidge when he was President … The President and Mrs. Coolidge were being shown [separately] around an experimental government farm. When [Mrs. Coolidge] came to the chicken yard she noticed that a rooster was mating very frequently. She asked the attendant how often that happened and was told, "Dozens of times each day." Mrs. Coolidge said, "Tell that to the President when he comes by." Upon being told, the President asked, "Same hen every time?" The reply was, "Oh, no, Mr. President, a different hen every time." President: "Tell that to Mrs. Coolidge."
The joke appears in a 1978 book (A New Look at Love, by Elaine Hatfield and G. William Walster, p. 75), citing an earlier source (footnote 19, Chapter 5).
The original experiments with rats applied the following protocol: A male rat was placed into an enclosed large box with four or five female rats in heat. He immediately began to mate with all the female rats again and again until eventually he became exhausted. The females continued nudging and licking him, yet he did not respond. When a novel female was introduced into the box, he became alert and began to mate once again with the new female. This phenomenon is not limited to common rats. The Coolidge effect is attributed to an increase in dopamine levels and the subsequent effect upon an animal's limbic system.
While the Coolidge effect is usually seen demonstrated by males—that is, males displaying renewed excitement with a novel female—Lester and Gorzalka developed a model to determine whether or not the Coolidge effect also occurs in females. Their experiment, which used hamsters instead of rats, found that it does occur to a lesser degree in females.
On simultaneous hermaphrodites
A 2007 study focusing on the Coolidge effect in simultaneously hermaphroditic species confirmed the validity of the Coolidge effect in freshwater snail Lymnaea stagnalis. Biomphalaria glabrata, another simultaneous hermaphrodite freshwater snail, does not exhibit sex-specific effects of partner novelty, and thus there is either no Coolidge effect in the species or no difference between the degrees to which the effect is expressed in the respective sexes.
- Reber, A. S. & Reber, E., The Penguin dictionary of psychology (3rd ed.), London: Penguin, ISBN 0-14-051451-1
- Brown, R. E. (1974), "Sexual arousal, the Coolidge effect and dominance in the rat (Rattus norvegicus)", Animal Behaviour 22 (3): 634–637, doi:10.1016/S0003-3472(74)80009-6
- Lester, GL; Gorzalka, BB (1988), "Effect of novel and familiar mating partners on the duration of sexual receptivity in the female hamster", Behavioral Neural Biology 49 (3): 398–405, doi:10.1016/s0163-1047(88)90418-9, PMID 3408449
- Pinel, John (2007), Biopsychology (6th ed.), Boston: Pearson Allyn and Bacon, ISBN 0-205-42651-4
- Carlson, N. (2013). Reproductive Behavior. In Physiology of Behavior (11th ed., p. 332). Boston, MA: Pearson Education.
- Dewsbury, Donald A. (2000) "Frank A. Beach, Master Teacher," Portraits of Pioneers in Psychology, Volume 4, p269-281
- Beach, F. A. & Jordan, L. (1956), "Sexual Exhaustion and Recovery in the Male Rat", Quarterly Journal of Experimental Psychology 8: 121–133, doi:10.1080/17470215608416811
- Wilson, J; Kuehn, R. & Beach, F. A. (1963), "Modifications in the Sexual Behavior of Male Rats Produced by Changing the Stimulus Female", Journal of Comparative and Physiological Psychology 56: 636–644, doi:10.1037/h0042469
- Fiorino, D. F.; Coury, A. & Phillips, A. G. (1997), "Dynamic Changes in Nucleus Accumbens Dopamine Efflux During the Coolidge Effect in Male Rats", Journal of Neuroscience 17 (12): 4849–4855
- Koene J. M. & Maat A. T. (6 November 2007) "Coolidge effect in pond snails: male motivation in a simultaneous hermaphrodite". BMC Evolutionary Biology 7: 212. doi:10.1186/1471-2148-7-212
- Häderer I. K., Werminghausen J., Michiels N. K., Timmermeyer N. & Anthes N. (12 October 2009) "No effect of mate novelty on sexual motivation in the freshwater snail Biomphalaria glabrata". Frontiers in Zoology 66: 23. doi:10.1186/1742-9994-6-23.