First, the Definition: What is Amusia?
Very informally, amusia can be defined as an inability to appreciate music.
Unfortunately, the problem of defining "amusia" precisely is almost as problematic as that of defining "music". If we consider music as being fundamentally a desire, then amusia would be defined by a lack of interest in music, whereas if we consider music as being fundamentally a skill, or a set of skills, then amusia would be defined by a lack of such skills.
Because people with a lack of interest in music usually have associated perceptual deficits, such as an inability to make fine pitch discriminations or an inability to recognise different types of music, there is a tendency to blur the distinction between lack of desire and lack of ability.
The confusion is worsened by what could be called musicism, i.e. the belief that music and interest in music is unconditionally a good thing, and therefore anyone with a lack of interest in music has something wrong with them which needs to be fixed, if at all possible.
But if we are going to be at all scientific about it, then we should distinguish concepts as basic as desire and ability from each other. For example, a scientist studying eating behaviour would certainly distinguish between the desire to eat a certain food and the ability to eat the same food.
For the Purposes of this Article, "Amusia" = Lack of Desire
For the purposes of this article, the main fact of interest about music is that it exists and that people put effort into creating it and appreciating it.
The scientific mystery of music is why people should put such effort into music, and what it is they get out of it that justifies, from the point of view of long-term reproductive success, the cost of that effort. To me, when I am studying this mystery, the most useful definition of amusia is one that refers to the condition of a person who does not, for whatever reason, put any effort into either creating or appreciating music.
This definition gives us the chance to avoid the musicist viewpoint, that is, we do not make any prior assumption as to whether music has some intrinsic value separate from the value that people attribute to music because they like music.
If we believe that music has a biological value (i.e. it increases long-term reproductive success of those who make it or listen to it), then we have to ask why it is that some people don't like music. If we believe that music doesn't have a biological value, then we have to ask why it that some (or actually most) people do like music. Since it is currently unclear as to whether music does or doesn't have such a value, we need to keep an open mind and be prepared to answer either of these two questions.
Two Types of Theories of Music
The question of the adaptiveness of music divides theories about music into two categories:
- Music is an adaptation in itself. The creation of music, or the appreciation of music, or possibly both, contribute to long-term reproductive success.
- Music is not an adaptation in itself. The appreciation of music is a side-effect of how the brain operates.
Each of these types of theory has its own unsolved problems, and therefore neither type of theory can be considered to have been ruled out as a possibility. (Also some current theories of music are stated sufficiently vaguely such that it is not clear which of these two categories they belong to.)
For the purpose of this discussion, I will call these two types of theory adaptive and non-adaptive.
Some well-known adaptive theories are:
- Sexual selection: Musical skill is a way of "getting laid".
- Group bonding: Music is a way of getting everyone in a society to come together and interact socially.
- Motherese: Music is a way for mothers to interact with their infacts. (This is one of those theories partly in the non-adaptive category, because we might consider that speech motherese is the primary mode of communication between mother and infant, in which case music is somehow a side-effect of the existence of this special type of speech.)
- Expectation/pattern recognition: Music provides a method for practicing useful perceptual skills.
There aren't so many non-adaptive theories, but two to mention are:
- Steven Pinker's "auditory cheesecake" theory: Music is a consequence of various perceptual preferences being combined into a single perceptual experience.
- My own "super-stimulus" theory: Music is a super-stimulus for one particular aspect of speech perception, one that we are not consciously aware of when perceiving normal speech, but which does affect our perception in some important manner. One variant of the super-stimulus theory is the "auditory super-cheesecake" theory – music is a super-stimulus so powerful that the human species has had to evolve resistance to it, and our current response to music is a reduced version of the very powerful effect that music had on our ancestors when it was first discovered.
The Problems of Adaptive and Non-Adaptive Theories
Each type of theory gives rise to different questions that must be asked. An adaptive theory must convince us that the alleged benefits of music justify the amount of effort put into making or listening to music. For example, rock music might get rock stars laid, but it seems that a whole lot of other people have to take a whole lot of interest in rock music just so that those rock stars can get laid.
A non-adaptive theory must tell us what is the "other thing" (or things) that music is a side-effect of. And also, because music has been around for a long time, it must explain why our spurious interest in music has not evolved away. For example, there must be some kind of lock-in between the "other thing" and the appreciation of music, such that evolution can't figure out a way to keep the first and discard the second.
What These Theories Have to Say About Amusia
Both adaptive and non-adaptive theories of music have something to say about the significance of amusia. Both types of theory suggest that amusia is a form of disability: with adaptive theories the disability lies in the lack of interest in music itself; with non-adaptive theories the disability must relate to the "other thing" that music is a side-effect of.
For example, if music helps you get laid, then amusia is a sexual disability. If music helps you socialize, then amusia is a social disability. And if music helps you bond with your infant, then amusia is a parenting disability.
If music is a side-effect of a motley collection of perceptual preferences, then amusia is a deficit in some subset of those preferences. And, if as I suggest, music is a side-effect of the perception of the "musicality" of normal speech, then amusia arises from a deficit in the perception of that aspect of speech (and there must be some consequent deficit in the cognition of speech, the exact nature of which depends on what it is that perceived musicality represents).
The Irrelevance of the Cause of Amusia (to this discussion)
Evolutionary questions about amusia exist somewhat independently of the cause of amusia. For example, the perception of music may depend on a complex inter-related set of brain functions, and a deficit in just one of those functions (for example fine pitch discrimination) may be the cause of amusia. But this does not change the questions that the different types of music theory must answer about amusia.
In particular, an adaptive theory of music must explain how amusia exists given that the amusic individual loses the alleged adaptive benefits of music.
A non-adaptive theory of music must explain how amusia exists given that the amusic individual loses at least one component of the "other thing" that music is a side-effect of. This requirement cannot be avoided by asserting that the lost component is relatively non-essential to the "other thing", because if it was, then the relative costs and benefits of this component would have resulted in everyone evolving the genes required for amusia.
Caveats
Although non-adaptive theories of music imply that amusia must be accompanied by at least one significant deficit in some brain function other than the actual creation and appreciation of music, there are some caveats:
- The relevant deficit may be probabilistic in its effect on brain development. In other words, some amusics may develop almost completely normally, using other aspects of perception to compensate for whatever deficit it is that accompanies their amusia. Other amusics may develop much worse than normally.
- The processes of selecting amusics for scientific study may select against precisely the deficits that one is hoping to find. To give an hypothetical example, if amusia sometimes causes a deficit which results in poverty, and the process of recruiting subjects for studies into amusia selects strongly for people living middle-class lifestyles, then the most interesting and relevant data is already eliminated from the study.
- The deficit may be relatively mild, and lost in the "noise" of human variation, unless careful statistical methods are used to measure the relevant mental functions of amusics against those of "normal" people.
- The deficit associated with amusia may mostly affect early brain development, in which case amusia with an onset later in life may not be accompanied by any deficit other than the amusia itself. In particular, cases of amusia caused by brain damage in adults may tell us little about what the "other thing" is that music is a side-effect of.
- The deficit may be dependent on circumstances in which a person lives and these circumstances may be different now from what they used to be. This is a version of the environment-of-evolutionary-adaptedness argument, i.e. X was adaptive when our ancestors lived the stone age hunter-gatherer lifestyle, but it isn't so obviously adaptive these days. To find the true deficit associated with amusia, we may need to study amusic individuals living in traditional stone age hunter-gatherer societies (if there are still any left that can be freely studied by curious music scientists).
Disability, or Evolutionarily Stable Strategy?
Looking for deficits associated with amusia, or treating lack of interest in music as a deficit in itself, is of course an example of the musicist viewpoint that I mentioned earlier.
But a significant fact about amusia is that the frequency is quite high, with estimates of at least 4% in the general population.
When I categorise 4% as "high", I mean that it could be considered too high for the occurrence of a "pure" disability. (I don't know if an exact percentage can be calculated, but 0.1% is a maximum typical frequency for a genetic condition that is purely a disability and has no compensating benefits.) In other words, to maintain genes causing amusia at that level within the population, there must be some compensating advantage. At the same time, there must be some selective pressure preventing the percentage from growing to some larger value. This is the logic of the Evolutionarily Stable Strategy or ESS.
The implication is that genes causing amusia have both advantages and disadvantages, and the balance between the two is a function of the prevalence of those same genes in the population. So, if everyone was amusic, then you would be better off with "normal" music perception, whereas if no one was amusic (and everyone had a "normal" interest in music), then you would be better off being amusic.
One obvious candidate for an advantage to being amusic is that the amusic individual does not waste time listening to music or making music, and their thoughts and emotions are not distracted by the presumably spurious emotional effects that music has on all the rest of us.
The strength of this advantage would be relatively independent of frequency of occurrence of amusia in the population – at least for low percentages, since for example the difference in exposure to music when 95% of the population is musical is going to be near identical to that when 97% of the population is musical.
So if that is the major advantage, and we want explain amusia as part of an ESS, we have to find some disadvantage of amusia versus normal musicality which is dependent on the frequency of occurrence of amusia in the general population.
A low percentage like 4% suggests a possible "free-rider" effect, i.e. if most people are musical, then this creates a cultural environment where the cost of not being musical is somewhat reduced. For example, if music perception is a consequence of a mechanism of world-view construction, then this may result in the widespread construction of reasonably sane world-views, such that there is less need to be selective when creating one's own world-view by absorption of the spoken assertions of other people. But if the percentage of amusic individuals rises, then the world-views become less reliable, and there is a greater advantage is using musicality perception to be more selective.