Prevalence of Synesthesia – about the difficulties of testing for synesthesia.

How many people resonate with synesthesia? Or in other words, how frequent is synesthesia in the overall population? How common is Synesthesia?

In a nutshell: the estimations for the prevalence of synesthesia range from 1% to 25%. Yes, this is kind of a large range. The honest answer is, that it is tricky to test for synesthesia. And is even more challenging to figure out the frequency of synesthesia. So science has not yet a clear answer to the question how common synesthesia is.

In this article, you will get an overview about this hot topic in the synesthesia research.

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“Diagnosing” Synesthesia is challenging

DISCLAIMER: If you have in mind to do a Synesthesia quizzes, you may do this before reading this paragraph. Click here to do a quick synesthesia quiz.

Diagnosing Synesthesia” is challenging. Even though there are different methods are available. The Gold Standard of “testing” for synesthesia is the consistency test that has been used widely over the last 20 years.

The consistency test works like this:

For most synesthetes, the colors of the letters remain the same over time. If you ask somebody with synesthesia twice for the color of a letter, the answer should be more or less the same. Even if the retest is done after a while. Non-synesthetes choose colors rather randomly. Scientists use this kind of test to check people for synesthesia. It is an established and refined method to do so.

These synesthesia tests are a useful and fun tool allowing a first insight into somebodies synesthetic abilities. But they have their limitations as well.

Screenshots of synesthesia tests. On the left the established synesthesia battery (, on the right our test. The principles are the same. Choosing colors for elements such as letters or numbers several time. Those who choose similar colors, may be synesthetic.
Screenshots of synesthesia tests. On the left the established synesthesia battery (, on the right our test. The principles are the same. Choosing colors for elements such as letters or numbers several time. Those who choose similar colors, may be synesthetic.

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Not all types of synesthesia can be tested easily

These consistency tests ask for the colors of letters and numbers. But not all types of synesthesias involve colors as a co-activated sense (also called concurrent).

For example, letter -> taste or letter -> personality synesthesia cannot be tested with this method.

Furthermore, there is emerging evidence, that the colors are not consistent for all people with synesthesia. These synesthetic participants would “fail” in such a test and not be classified as synesthetes.

Traditionally, the definition criteria for synesthesia is to have consistent color-letter associations. And hence those who have consistent color-letter associations are synesthetes. But this may also be a self fulfilling prophecy.

You see, diagnosing Synesthesia is tricky. Therefore it is also not a surprise, that we do not have a good answer yet to the question: How frequent is synesthesia?

The Prevalence of synesthesia – how common is it?

As mentioned, the prevalence of synesthesia is estimated to be somewhere between 1% and 25%.  

Yes, you may be now disappointed by this very imprecise answer, since this is quite a wide range. So let me explain.

The estimated prevalence of synesthesia has continuously risen the past 30 years.


Early prevalence of synesthesia estimations were way too low

In the 80’s , when Richard Cytowic started to investigate synesthesia, he assumed that it is a very rare phenomenon. One out of 2000 was his estimation of the prevalence of synesthesia.

But this was not really based on empirical data. He came up with this number by putting an advertisement into the newspaper seeking for synesthetes. Afterwards he calculated the estimated prevalence through the number of synesthetes that responded to the advertisement and the edition size of the newspaper.

So let’s get back to the empirical data and review briefly the research about the prevalence of synesthesia.


With new studies the estimations of the prevalence of synesthesia went up continuously

With the emergence of the consistency tests, Simner and Ward were the first to do investigation in overall population. They came out with an estimation of around 1% of grapheme-color synesthetes in overall population. Random visitors of a museum did the test. The resulting estimation was that around 1% of the test person had grapheme color synesthesia. around 5% had at least one kind of synesthesia.

This may be an underestimation. The study was done very conservatively. This means the researchers wanted to avoid classifying non-synesthetes as synesthetes. Participants had to fill out a questionnaire and anyone who initially stated not having synesthesia was excluded from any further process and classified as a non-synesthete. This procedure is critical. Some people with synesthesia may just not be aware of it.. For this reason the prevalence of this study may have been low.


Different studies come up with different prevalence of synesthesia

Another study had a look at art school students in Switzerland. They found that the prevalence of grapheme-color synesthetes is higher in artistic schools with around 7% compared to 2% in control groups.

A new study with student in Canada and Czech found that around 2% resp 4.4% had at least one type of synesthesia. They claim themselves that this may be an underestimate.

On the other side, one study proposed that 20% of population had Calendar Synesthesia.

Another study proposes that 22% could have a motion -> sound synesthesia. Meaning, when they see objects moving, that they hear something.

A dutch study from 2016 used questionnaires, but not the typical consistency test. They came up with a 25% estimation. This would mean, every fourth person on this planet has at least one type of synesthesia. But the authors of this study admit themselves, that it is an overestimation.

You see, there is not yet an agreement how frequent synesthesia is exactly. There is definitively a trend moving away from “a rare phenomenon” towards something that may be much more common than was assumed 30 years ago.

But why is it so hard to come up with accurate estimations?


The Balance between underestimation and overestimation

Scientists try to be conservative in these matters

A non-synesthete that is classified as synesthete would be a false-positive. A synesthete falling through the net and being classified as a non-synesthete would be a false-negative.

In prevalence studies, researchers try to avoid false-positive results but neglect the false-negative ones.

In other words to be conservative means, it is better to have a too low prevalence estimation than too high.


Unaware synesthetes are not accounted for

One of the main problems is, that in such tests unconscious synesthetes are neglected.

In many studies, they asked the participants if they have synesthesia. If they said no, then the “test” was over. Not further proceeding and they were classified as non-synesthetes.

Given that there are many unaware synesthetes that do not know about their synesthetic abilities the data gets distorted. Obviously unaware synesthetes won’t report on, or claim to be synesthetic. So, there is a considerable amount of synesthetes in these prevalence studies, that probably were classified as non-synesthetes. Which would lead to low prevalence estimations.

We reported about this in our congress paper. It was just in 2017 that the first study reported about this phenomenon of unaware synesthetes.


Can there be a sharp line between synesthetes and non-synesthetes?

The consistency tests measure how similar the colors are that you choose for a letter or a number. Doing such a test, you get a score. Depending on that score, participants are classified as synesthetes or non-synesthetes. But who sets the threshold of these scores?

Depending on how to set the boundary or the score threshold, the prevalence estimations are higher or lower. If you establish a rigid threshold, less participants are diagnosed with synesthesia. If this threshold is set rather slack, more participants are synesthetic. You see the problem?

The line between synesthetes and non-synesthetes is very sharp in such consistency tests. But this can be a problem!

According the score of the synesthesia test, participants are classified as synesthetes or not. You see in the grey histogram, that there is a sharp line. This is critical because there is an overlap. An unclear range (yellow) could be a solution.
According the score of the synesthesia test, participants are classified as synesthetes or not. You see in the grey histogram, that there is a sharp line. This is critical because there is an overlap. An unclear range (yellow) could be a solution. (Adapted from Simner et al. 2006)


Score tresholds were defined by aware synesthetes

This synesthetic “score threshold” was set according the scores of aware synesthetes. If participants score similar to aware synesthetes, then they are classified of having synesthesia.

But there is a problem. Aware synesthetes may be more consistent in such tests, than an unaware synesthete. For this reason, some synesthetes may fail in such tests too.

It is kind of a problem that there should be a sharp line between the scores of people with synesthesia and without. Sometimes it is possible to classify somebody as non-synesthete or synesthete. But sometimes it is very difficult. Therefore, we think there should be a third category: “unclear”.

So, you have seen, it is not easy to test for synesthesia and hence it is also not easy to come up with a good prevalence estimation. Different studies come up with different estimations.

We estimate that the prevalence of synesthesia is around 10%

More studies are needed and it won’t be easy to figure out the exact prevalence. Our guesstimation is 10%. But do not cite us as scientific fact, please. It is just what we think according our experience.

What is actually much more important:

Synesthesia is very common. You and some of your friends and family members may have synesthesia too. So, be open to talk about synesthesia. You will be astonished, how many others are out there too.


What to do next?

Do a quiz: Even though synesthesia tests have its limits, they are fun and insightful.


Increase your mindful synesthetic awareness: Listen to our exercises and become aware of synesthesia


Read more: Maybe you want to learn more about why some people are unaware of synesthesia? Or what types of synesthesia there are? Here on the blog you find more information about synesthesia.

Speak about synesthesia: Ask a friend or a family member about the colors of their letters and numbers.