Why Listening to Music Is Pleasurable (my guess)
Listening to music is pleasurable because it just happens to trigger pleasure responses that evolved for other reasons, similar to the way that a recreational drug can mimic a pleasure hormone. Here I suggest that one such pleasurable response is the satisfaction that results when we accurately predict the timing of periodic events. Another is the relaxation that results when things appear to be in order. Yet another is the excitement that results when something unexpected happens. And finally, complexity helps us maintain focus on music, thereby heightening its effect.
Side note: The reason that some people are inclined to practice making music is another thing altogether. The inclination to spend time and effort developing a skill that's not crucial to survival or reproduction, but which can confer high social status, is more likely a result of sexual selection. The discussion here is instead about why hearing music is pleasurable in the first place, which only then leads to high social status for those who can create it skillfully.
Satisfaction from Accurate Timing
Certain basic skills that are important for survival require good timing. To run as efficiently as possible, for example, it is important to be able to predict precisely when your foot is going to strike the ground. This allows you to make the appropriate adjustment to your muscular effort immediately on impact, rather than first waiting to feel the impact and only then making the adjustment. A momentary feeling of satisfaction when we accurately predict the timing of our footsteps has therefore been selected for, because it motivates us to hone that skill that helps us to escape predators, for example. But since natural selection tends to overgeneralize as it stumbles upon behaviors that increase our reproductive rate, it is likely that it selected for a general satisfaction response whenever we accurately time regularly-spaced events that occur on the order of a few times a second, as with footsteps in running.
Most music exploits this satisfaction response by having musical events that occur at a steady beat that has a frequency similar to the footsteps of running (or walking). A musician who has steadier timing is more highly valued because the exact time of each note that they play can be predicted more accurately. A listener can enjoy the satisfaction of "feeling the beat" without doing any work, making musical enjoyment something of a free lunch. Moving to music in some oscillating way, though, can heighten the effect by using your body as a physical pendulum that helps you feel the timing more accurately; and so physical movement from toe-tapping to dancing co-evolved with musical appreciation. Dancing, while requiring more physical work, may trigger the response in the most intense way as you feel the beat of both the music and your body movements together.
Feeling the beat while running can also help us maintain a steady gait. When threatened by a predator, in particular, the fear response helps us to flee more quickly. But it can also disturb the general rhythm of our running, causing us to stumble and fall and get eaten. The ability to maintain our composure even during intense excitement has therefore been selected for. Getting into the groove of a song so that you can "lay back into the beat", no matter how exciting the music is, exploits this related satisfaction response. That's why it's particularly annoying when a musician rushes ahead of the beat, such as a drummer who comes out of an exciting fill a little too soon.
In addition to the importance of timing each step well while running, it is also important to be able to accurately time less frequent, less regularly-spaced events, such as jumping over an obstacle or moving onto a different kind of surface that may require a somewhat different gait. I've noticed when I run on the street that I get a sudden sense of which foot is going to jump onto a curb when I'm six or eight steps away, and I can then adjust my step as needed to hit the curb just right on the anticipated foot. So there may be a separate selected-for satisfaction response for the timing of such less frequent events. In music, chord changes are one thing that exploits this response, with the particular satisfaction that results from anticipating an upcoming chord change and then feeling it hit on the expected beat, similar to landing on a different surface on the expected footstep.
It may seem dubious that running is such a key influence. But humans are one the most efficient species at long-distance running, and so it apparently is integral to our humanity. Maybe running is the key source of the satisfaction that we get from accurate timing, or maybe it is just one example of a general rule.
Satisfaction from Scratching Every Spot
Satisfaction is also experienced whenever a melody emphasizes a pitch that hasn't been emphasized recently. This particular effect reminds me of scratching an itch. When scratching off tiny parasites, it's important to cover every location on the skin in order to remove all of them, and not to keep scratching one spot repeatedly until the skin becomes irritated. When we hear a pitch, it stimulates a patch of inner ear hair along the cochlea that responds to that particular pitch. So hearing different pitches is similar to "scratching" different spots along the cochlea. Perhaps hearing a pitch that hasn't been played for a bit triggers a selected-for response for "hitting every spot" when scratching, whereas harping on one note is irritating because it's over-scratching one place.
Relaxation from Sensing Order
When you survey your surroundings and see no hostile intruders or conflict within the group that would warrant being on alert, you are then free to relax and enjoy yourself. This has led to a general tendency to relax and feel good whenever we sense that things are in order and everyone is getting along.
In music, order is demonstrated in a variety of ways, and so listening to music can produce a sense of order and the associated comforting feeling. Order is demonstrated at each moment, such as by the harmonizing notes of a chord, and also over time, such as by a predictable rhythm over the short term and by repeating motifs or themes over the longer term.
Chords are sensed as orderly (non-dissonant) when they are made up of notes whose frequencies are simple ratios to each other, such as five to four for a major third, just as overtones have simple ratios with a note's fundamental frequency. It may seem rather arbitrary that we should perceive order when a second note mimics an overtone, especially since the overtones of the two notes will still "clash" by not having simple ratios between them. But order can be perceived from most any pattern that appears highly unlikely to be a coincidence. The ear can recognize that a second note, while too loud to be an overtone of a first note, is nevertheless the same frequency as an overtone of the first note, thereby implying some sort of intentional cooperation (also known as harmony) between separate sound sources, leading to the comforting feeling that everyone is getting along harmoniously, as it were.
Excitement from Unexpected Events
While relaxation is all well and good, it can become rather boring. If occasional unexpected events occur, though, it can excite you and thereby hold your attention. Events such as an atypical chord change serve this purpose in music.
This may seem to conflict with the relaxing effect from sensing order. But music can be relaxing most of the time even as it perks you back up now and then. Occasional possibly threatening surprises are not especially unpleasant or unhealthful, and can even create a sense of being more alive. It is continuous low-level anxiety instead that is unpleasant and harmful. The overall sense of order in a song eases continuous anxiety, even as unexpected events wake you back up now and then, to strike a compromise between general relaxation and occasional excitement. And the small jolts are not bad especially when you then discover that they are actually part of a larger order after all. (And if you are more in need of excitement than relaxation to begin with, then perhaps more and bigger jolts are in order.)
Occasional disturbances can even increase the ultimate relaxation level, by building tension that creates the potential for a subsequent relaxing rebound that overshoots the original state of moderate relaxation. An example of this is the pronounced relief that you feel when a dissonant chord "resolves" to a consonant one. This is similar to intentionally tightening a muscle so that it can then be be relaxed more effectively than by simply trying to relax it directly.
Complexity to Increase Focus and Heighten the Effects
To maximize the pleasurable effects of music, it's important to maintain as much mental focus on it as possible. This is problematic because music is not important at all to survival and reproduction, and so our minds tend to wander over to more crucial concerns. Fortunately, various aspects of music have come along to help us maintain focus. One is the excitement of unexpected events as mentioned above, because a surprise causes us to focus on the source of the surprise to see if it indicates danger.
Another way that music helps us maintain focus is by adding complexity. Complexity can engage the more intellectual level of brain activity that may get bored while the more primal part of the brain is feeling the beat. Complexity in the rhythm can even synchronize those two levels of brain activity with each other, as one is intrigued by the complexity while the other feels the basic beat behind it.
A central type of complexity in music is patterns, such as variations on a motif. Pattern-matching is a crucial mental task that enables us to predict what is going to happen next and how best to respond. So a strong tendency to look for patterns anywhere has been selected for, and patterns in music trigger this tendency. This engages the higher-level puzzle-solving part of the brain, keeping it focused on the music to heighten the pleasure at all levels.
Subverting the Drudgery of Life that Has Evolved Only to Reproduce Genes
Natural selection leads us to work constantly to maximize our reproductive rate, rewarding us with pleasure only for putting in the work. Music subverts that dictate by exploiting pleasure responses that evolved for other reasons, providing pleasure for free. An oasis of pure joy amidst the daily grind of life.
2011-12-18 (last edited 2015-03-03)
[ I came up with this thought experiment after deciding that Daniel Levitin's theory (in the book "This Is Your Brain On Music") that musical enjoyment is due to sexual selection really applies only to practicing the playing of music. The enjoyment of music must have there to begin with in order for the ability to make music well to be valuable. And then on reading Steven Pinker's argument in the book "How the Mind Works" that music happens to trigger brain responses that exist for other reasons, his details were unsatisfying to me because the brain responses that he describes are not pleasure responses and so do not explain why music is pleasurable. So I began to ponder how music could trigger existing pleasure responses. ]
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