Al Cruz/UnsplashImagine a scene from the movie Jaws, with the great white shark closing in on another helpless victim. The iconic semi-tone pattern builds and your heartbeat rises with it; the suspense pulls you further to the edge of your seat.Now picture that scene without the score. Much of the tension evaporates.Maybe it’s a heartfelt pop ballad or a suspenseful soundtrack. If you are my age, it might be the Friends theme song, forever associated with the (largely unfulfilled) hope for sharing apartments with mates and growing old together in a blissful acceptance of one another’s limitations. Music is a powerful force to induce and pre-empt all kinds of emotions in us.But how do so many different combinations of rhythm, harmony and melody trigger such profound reactions?The categorical approachSwedish music psychology researcher Patrik Juslin proposed the most popular explanation of music’s ability to trigger emotion.He identified eight key mechanisms under the acronym BRECVEMA. The categories begin with more fundamental connections:Brain stem reflexes – maybe a movie jumpscare moment or another sudden, frightening sound triggering a pre-conscious response. Evolution programmed these reactions into the brain over thousands of years in order to influence arousal levels and initiate the necessary emotional response.Rhythmic entrainment, like the tendency to tap your foot to the beat; the benefits of moving in time together have been critical to human survival and evolution.Then, the listings become increasingly complex:Evaluative conditioning in the fashion of Pavlov’s dog. After years of watching and cultural references, we hear the Jaws music and automatically feel tense.The contagion effect, wherein we feel the emotions we perceive in the music. Lyrics aren’t necessary; the Peanuts cartoon’s signature tune, for example, strongly conveys childhood wonder and freedom without any words.The visual imagery many people experience when listening to music, imagery which is often tied to some deep emotion.Episodic memories, when hearing certain music brings up recollections of a past event. Music therapists can monitor the emotional reactions people have when unexpectedly reminded of particular situations, be they positive, negative or both. The therapists then use their expertise to support people in processing these resulting emotions.From there, Juslin’s model gets more technical and music theory-based:Musical expectancy, when we anticipate the resolution of a chord or phrase. This is something you might feel rather than consciously notice. Take My Heart Will Go On: a delicate tension builds through the chorus, before finally resolving as Celine Dion sings the final line of the section and listeners are put to ease.Aesthetic judgements, closely related to the ways we experience pleasure, are our personal emotional responses to how beautiful (or not) we consider a piece of music. Read more: Different songs for different days: why it’s important to actively choose the music for your mood It makes sense that a theory using the brain to explain otherwise indescribable relationships would be popular. It provides a level of objectivity to what is, in essence, a purely subjective and non-generalisable experience. Celine Dion keeps listeners on tenterhooks before the chorus comes to a beautifully satisfying resolution. Is it just about neurological pathways?Evolutionary theories suggest music and emotions are connected because of the inherent musicality we are each born with, essential to our ability to develop relationships and flourish.Parent-infant interactions often have musical aspects to them, described as:pulse, a shared tempo, where infant and carer move in time together and synchronise to one underlying beatquality, the character and melodic interplay of voices and movements, mirroring one another in dynamics and timbrenarrative, the tendency for the same phrases, gestures and movements to be repeated on the same pitch and pace over time.When responding to musical sounds, babies are also able to recognise musical phrases even when they start on a different note.Subsequently, however, other learning and our limited brain capacity mean this ability is buried deep, so it rarely translates to perfect pitch or other forms of music theory knowledge that underpin Mozart-like genius. All of us are born with an inherent musicality. FamVeld/Shutterstock This baby-talk theory may be the most intimate and emotion-based explanation for why music affects us so strongly – it was designed to enhance our emotional bonds with others. When adults coo and dance with babies, they are being musical, meaning emotional reactions to music are implicit in human nature.Cognitive developmental theorists like Steven Pinker have opinions firmly in contrast to this. Pinker calls music “evolutionary cheesecake”, functioning only to tickle the senses and serving no evolutionary purpose.Pleasure for purposeCultures across the world have long acknowledged the healing power of music. Sound healing practitioners in India and China, for example, point to ancient traditions of healing and draw correlations between recovery from illness and certain tones, scales and chants. Some suggest the vibrations of different tones can serve specific purposes.In the West, the idea of emotional differences between major and minor scales still has public traction even though its academic credibility hasn’t really extended in the past 100 years. None of these concepts have been used in the modern practice of music therapy, but they do reflect assumptions many people hold about how music works.Instead, a fundamental principle of music therapy is based on how each person’s unique connections with music shapes their emotional reactions. What moves your sibling to tears might leave you cold, for example. It always depends on a range of conditions – historical, cultural and personal.Cultural upbringing, simple song-like phrases from infancy and our own unique musical preferences and behaviours all shape these connections. They’re powerful, but they sure ain’t simple.Katrina McFerran has received funding from the Australian Research Council to investigate music and emotions. She is affiliated with the Australian Music Therapy Association.