What is the rarest type of synesthesia?

There’s no answer to that question! It’s impossible to say which type it is, for several reasons. But it is certainly NOT lexical-gustatory, as some websites erroneously state (see footnote 1 about the reasons for this).

In brief...

In general, the types of synesthesia with a non-visual concurrent (olfactory, gustatory, tactile, etc.) are much less common than those with a concurrent of colour.

No specific type/subtype/manifestation of synesthesia can be called “the rarest type” because there are so many that only a few people have (and it’s impossible to calculate exactly how many people have them), and there are also types that logically exist but have never been reported by anyone.

Also, there is no universal system for determining what actually constitutes “a type of synesthesia”.

What might some candidates be for a supposedly "rarest type"?

Here are some types or sub-types that are very uncommon (with links to the page containing information on them and reports by people who have them):

Grapheme-smell/taste (each letter and/or number has a smell or taste. This type shouldn’t be confused with lexical-gustatory synesthesia, which is word-taste and is much more common as probably around 3% or more of synesthetes have it. Lexical-olfactory, where words have smells, is rarer than lexical-gustatory.)

Tactile-olfactory and tactile-gustatory (on touching different surfaces, the synesthete consistently perceives a particular smell or taste)

Perceived emotion-touch sensations (the perception of different emotions in other people creates a specific, consistent tactile sensation in a particular part of the body)

Conceptual-auditory synesthesia is a very uncommon type, and an excellent example of a very rare or perhaps unique manifestation is hairstyles-sound.

However, we cannot really say that the above types of synesthesia are the least common, as cases have been described and confirmed. The ones that really would be the rarest are those for which no case has ever been reported. Some examples of types that probably exist but which as far as I know no-one has reported yet are:

Kinetics-smell and kinetics-taste (different movements of one’s own body create the consistent perception of a smell or taste) (... but take a look at the comment of June 13, 2024 below to see a very interesting description of just this!)

Grapheme-tactile (the concept of each letter or number produces a specific, consistent tactile sensation in a particular part of the body)

A problem with calculating the rarest types: the sample size

It is difficult to estimate the frequency of the most uncommon types of synesthesia because not enough people have them to provide a sufficiently sized sample population for testing. Quite reliable prevalence figures exist for the more common types of synesthesia, because rigorous scientific methods have been used to assess many hundreds of people after verifying the genuineness of their synesthetic experiences using techniques such as the Battery Test. But for the more uncommon types the reliable samples are very small, and in the case of the very rare types they are practically zero and there are simply no figures available: all we can do is guess.

But what does a “type of synesthesia” consist of anyway?

One of the problems in attempting to define what the rarest synesthesia types might be is in classifying what actually constitutes a “type of synesthesia”: how should we subdivide? Breaking them down into main groups – auditory-visual, olfactory-tactile, and so on – is not the same as finely subdividing them by particular manifestations into types like “decades-to-colour”, “weekday personification”, “morpheme-colour” or “novels I’ve read-spatial location”.

If we subdivide by overarching groups, the most common synesthesias would be those that have a colour perception as their concurrent, and the least common would probably be those with a tactile or olfactory/gustatory concurrent (see footnote 2).

A large-scale research study by Novich, Cheng and Eagleman, Is synaesthesia one condition or many? A large-scale analysis reveals subgroups (2011) proposed 5 clusters of synesthesia types:

- coloured sequences (month-colour, grapheme-colour, etc.)
- coloured sensations (pain-colour, for example)
- music-colour
- synesthesias with non-visual concurrents (evoking smell, sound, touch, taste, etc.)
- spatial sequences

The least common cluster would be non-visual concurrents. Talking about clusters rather than specific types, subtypes or manifestations of synesthesia is a much more practical way of trying to determine what might be the “least common synesthesia type”.

On this page of the Tree: List of types of synesthesia by prevalence, the furthest column to the right shows a list of the types and manifestations of synesthesia that could be the rarest, with an incidence of less than 1% among the synesthete population. As you can see, they are very numerous: much more numerous than the more common types. I’d also like to clarify that this is only a rough approximation to the real situation, as apart from there being no reliable figures available for these types some of them might also be more widespread than was formerly believed (and they might jump to a higher column when more cases are reported).

Weak synesthesias

It should be borne in mind that many synesthetes have only very mild manifestations of a particular type of synesthesia or have simply not focused on it enough to realise that they have it, and this also influences the number of reports and figures available to researchers. In the past, as a result of this effect and the fact that synesthesia was hardly talked about (and up to about 1990 or 2000 scientific knowledge on the subject was much more limited), it was generally believed to be an extremely rare phenomenon, although it is now known that this is not the case, with a commonly accepted figure of almost 4% of synesthetes in the population. When a type of synesthesia begins to be discussed, it is commented in the dedicated groups and forums in the social networks and more people appear who can talk about their own experiences with it. But the rarer types of synesthesia are less talked about, precisely because they are rarer… and logically it is more difficult for them to come to light than in the case of more common types.

Synesthetic concurrents: mixed or on the rocks?

Another effect that complicates the possibility of counting cases of certain types of synesthesia is that some concurrents tend to occur jointly with others, making them more difficult to classify. Grapheme-temperature synesthesia is an example of this: for many synesthetes, temperature is probably an integral part of their grapheme-colour synesthesia, but it wouldn’t occur to them to say that they have grapheme-temperature synesthesia. If we only count the people who experience a temperature concurrent without an associated colour perception we would have one figure, while if we counted all grapheme-colour synesthetes with this additional temperature perception we would have another, much higher, one. Which would be correct? Another example would be grapheme-smell or grapheme-taste synesthesia, where the smells and tastes could be merely an accompaniment to the main perception of colour.

Who has the rarest forms of synesthesia?

Whatever the case, it is interesting to note that the most uncommon types of synesthesia tend to be experienced by people with a very high “synesthetic disposition”. This means that they have many different types, often strongly expressed. This phenomenon tends to occur more frequently in people on the autism spectrum, although not exclusively so. If someone has only one subtype or manifestation of synesthesia, or very few, it is very unlikely that this type would be precisely one of the rarest. If that seems to be the case, the phenomenon they are experiencing is probably not a type of synesthesia.

Footnote 1:

It is reported by Sean Day that 2.89% of 1,143 synesthetes interviewed had lexeme-to-flavour correlations. A figure of “less than 0.2% of synesthetes” can sometimes also be found, as in the important study conducted by Julia Simner et al. in 2005 on a sample of 500 members of the general population no-one reported it at all. However, accounts of lexical-gustatory synesthesia can be found without excessive difficulty, and it is certainly not “the rarest type of synesthesia that exists” as some websites currently claim. Until now there has been no standardised method for determining it, although work has recently been done on a system for diagnosing this type of synesthesia so the figures may eventually prove to be higher.

Footnote 2:

Emotion (as a single concurrent rather than a by-product of the synesthetic experience in general) could be considered an extremely rare concurrent as there is only one type accepted as synesthesia at the present time that has emotion as a concurrent: tactile-emotion synesthesia. However, it is a special case which perhaps should not even be considered a type of synesthesia at all, and it is easy to confuse an “emotional synesthesia” with many other phenomena that evoke emotions but are not considered types of synesthesia (frisson, misophonia, empathy, music causing emotion, etc.).

This page last updated: 16 June 2024

This Tree post was published on 13 July 2022

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