How many times does the chorus repeat in your favorite song? And, take a moment to think, how many times have you listened to it? Chances are you've heard that chorus repeated dozens, if not hundreds, of times, and it's not just popular songs in the West that repeat a lot. Repetition is a feature that music from cultures around the world tends to share. So, why does music rely so heavily on repetition? One part of the answer come from what psychologists call the mere-exposure effect. In short, people tend to prefer things they've been exposed to before. For example, a song comes on the radio that we don't particularly like, but then we hear the song at the grocery store, at the movie theater and again on the street corner. Soon, we are tapping to the beat, singing the words, even downloading the track. This mere-exposure effect doesn't just work for songs. It also works for everything from shapes to Super Bowl ads. So, what makes repetition so uniquely prevalent in music? To investigate, psychologists asked people to listen to musical compositions that avoided exact repetition. They heard excerpts from these pieces in either their original form, or in a version that had been digitally altered to include repetition. Although the original versions had been composed by some of the most respected 20th century composers, and the repetitive versions had been assembled by brute force audio editing, people rated the repetitive versions as more enjoyable, more interesting and more likely to have been composed by a human artist. Musical repetition is deeply compelling. Think about the Muppets classic, "Mahna Mahna." If you've heard it before, it's almost impossible after I sing, "Mahna mahna," not to respond, "Do doo do do do." Repetition connects each bit of music irresistibly to the next bit of music that follows it. So when you hear a few notes, you're already imagining what's coming next. Your mind is unconsciously singing along, and without noticing, you might start humming out loud. Recent studies have shown that when people hear a segment of music repeated, they are more likely to move or tap along to it. Repetition invites us into music as imagined participants, rather than as passive listeners. Research has also shown that listeners shift their attention across musical repetitions, focusing on different aspects of the sound on each new listen. You might notice the melody of a phrase the first time, but when it's repeated, your attention shifts to how the guitarist bends a pitch. This also occurs in language, with something called semantic satiation. Repeating a word like atlas ad nauseam can make you stop thinking about what the word means, and instead focus on the sounds: the odd way the "L" follows the "T." In this way, repetition can open up new worlds of sound not accessible on first hearing. The "L" following the "T" might not be aesthetically relevant to "atlas," but the guitarist pitch bending might be of critical expressive importance. The speech to song illusion captures how simply repeating a sentence a number of times shifts listeners attention to the pitch and temporal aspects of the sound, so that the repeated spoken language actually begins to sound like it is being sung. A similar effect happens with random sequences of sound. People will rate random sequences they've heard on repeated loop as more musical than a random sequence they've only heard once. Repetition gives rise to a kind of orientation to sound that we think of as distinctively musical, where we're listening along with the sound, engaging imaginatively with the note about to happen. This mode of listening ties in with our susceptibility to musical ear worms, where segments of music burrow into our head, and play again and again, as if stuck on repeat. Critics are often embarrassed by music's repetitiveness, finding it childish or regressive, but repetition, far from an embarrassment, is actually a key feature that gives rise to the kind of experience we think about as musical.