Why do humans respond to certain kinds of music the way they do?

Some of the most elementary musical motifs are universal because they express the structure of the universe.

• The notes of the pentatonic scale have frequencies in the ratio of small integers: C/D/E/G/A = 8/9/10/12/15. (This is two octaves below the lowest octave used in music because our hearing range does not extend that far down, but the ratios are the same in any octave.) If these notes are produced in quick sequence, the simple interference patterns they create in our eardrums are interpeted by our brains as melody.
• If two notes have frequencies in the ratio of extremely small integers: G/C = 3/4, our brains interpret this broader interference pattern as harmony. (I've gone down to an even lower octave to simplify the arithmetic.)
• Percussive sounds that repeat in a consistent, regular pattern, like the footsteps of a man walking or running, are interpreted by our brains as rhythm.

This combination of melody, harmony and rhythm is the basic definition of music. Rhythm is the only one of the three that occurs in inanimate nature, since winds and tides can cause repetitive percussive sounds, such as a shutter creaking back and forth. Animate nature, especially songbirds, can create melody. However, except for rare coincidences, harmony is only produced by humans who are deliberately creating music.

These simple principles of music are universal to all humans. We all respond to melody, harmony and rhythm. But as our cultures evolve, we develop styles of music that our culture responds to, but may not be immediately comprehensible to another culture. Paleolithic people, for example, discovered the pentatonic scale; a flute made from a mammoth tusk 35,000 years ago has holes bored in the perfect positions for that scale. But when the few surviving paleolithic tribes were exposed to European music with its myriad complex scales in the colonial era, they were taken aback. Similarly, many of us Westerners find the scales used in Arabic and Japanese music a little off-putting.

African people had developed music with intricate rhythmic patterns, which they brought with them to North America when they were captured and sold as slaves. One of the few "respectable" occupations open to slaves and freedmen was music, and many of them were sought after to play and sing at dances and other social events. But they discovered that if they used the rhythms of their ancestors, the white people grimaced and walked off. They had to stifle themselves and stick with the much simpler rhythms of traditional European music. (Even today, I can always distinguish African immigrants from Afro-Americans when a bar band is playing Led Zeppelin's "Kashmir" with its polyrhythmic structure: only the Africans can follow that beat and dance to it.)

At the turn of the 20th century, Afro-Americans had developed a new rhythm called "ragged time," which was shortened to "ragtime." It is heavily syncopated, meaning that the beats do not fall in a simple, predictable pattern. Many of the more adventurous Euro-Americans were fascinated by it, and made it their mission to understand it and learn to dance to it. But many others could not stand to hear it, so bands had to be prepared to switch repertoires to suit their audience.

The rhythm of ragtime and the modality (choice of scale) of Delta Blues (which was made possible by the invention of the steel-string guitar, IMHO the most important artifact of the Industrial Revolution

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) were the formative components of jazz. Throughout the early 20th century there were tens of millions of Americans and Europeans who could not stomach the sounds of jazz because they found them dissonant and jarring.

Eventually, jazz settled into the form of swing, and by the 1940s virtually all Americans had become adjusted to its somewhat tamed-down, predictable syncopation and its blues modality. But then in the 1950s jazz was fused with country music as a new generation of children (the "Baby Boomers") compulsively distanced themselves from the culture of the people who had just killed off three percent of the world's population and half of its Jews and deployed nuclear weapons against civilian targets (i.e., their parents)... and embraced rock'n'roll. In addition to the jazz and country elements, rock also featured loud, angry singing, lyrics about sex and rebellion, and syncopation so intricate that many adults couldn't figure out how to tap their feet to it.

Today those older people have either died off or adapted, and almost all American music is rock and roll (with offshoots like hip-hop and reggaeton). Soon a younger generation will get tired of it and invent something new, which will send all of us rock'n'rollers who are now old screaming for the exits.

So the answer to the original question is: Aside from the basic elements of melody, harmony and rhythm, the effect of music on people is learned, and passed down to later generations. So different cultures respond to different types of music... in different eras.

 

Other animals have been found to be responsive to music. I don't remember the species anymore, but several years ago someone experimented with a small, social, arboreal mammal. They had to A) speed up the music to be compatible with its natural movements, and B) increase the pitch by several octaves to match its range of hearing.

After a little tweaking of the parameters, they found the guys sitting on a branch and bouncing back and forth to the beat.