I’m sure President Coolidge would have been thrilled to have this idea named after him… It all goes back to an unverified story about him and his wife, Grace Coolidge, being shown separately around a government farm. When Mrs. Coolidge came across a rooster going at it with the hens, she asked the attendant how often he did that every day. The attendant replied, “Dozens of times each day.” Mrs. Coolidge then said, “Tell that to the President when he comes by.” Upon being told, the President asked whether it was the same hen every time. The reply was, “Oh, no, Mr. President, a different hen every time.” The President then said “Tell that to Mrs. Coolidge!”
So you get the idea. The Coolidge Effect is a phenomenon seen in mammals where males (and apparently females to a lesser extent) show renewed sexual interest if they are introduced to new sexual partners. This effect has been verified with rodents. This Journal of Neuroscience article describes the Coolidge Effect in greater depth, but the general idea is illustrated in the graph below:
Normally, every guy experiences what is called a post-ejaculatory refractory period after an orgasm. Translation: he needs a break after sex before getting it on again. But that period of time gets massively reduced with a novel sexual opportunity.
What is perhaps most interesting about this phenomenon is how it applies to porn. Gary Wilson of YourBrainOnPorn discusses how watching porn – despite the passiveness of the experience – is actually like having a new sexual opportunity with each new clip. He describes why this is not beneficial to the user or potential real-life partners in the talk below: