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Synesthesia: Types, Causes and How Common It Really Is

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Published: 3 Jul 2011 - Updated: 1 Nov 2026
Publication Type: Informative

Table of Contents:
Synopsis - Definition - Overview - FAQs - Insights, Updates - Related Content

Synopsis

This information explains synesthesia, a neurological trait in which one sense automatically triggers another, so that a person may see colors in music, taste words or perceive letters and numbers as colored. It corrects several long-repeated myths using population research published in Perception, which found that about 1 in 23 people have some form of synesthesia and that it affects men and women at similar rates, rather than being rare and mostly female. The article covers projecting and associative forms, the many reported types, Dr. Richard Cytowic's diagnostic criteria, acquired synesthesia after blindness or brain injury, and the higher rate of synesthesia among autistic adults. It is useful for synesthetes who wonder whether their experiences are normal, for parents and teachers of children who describe unusual sensory links, and for people with disabilities, seniors and caregivers who want to know when a new sensory change is worth raising with a doctor.

At a Glance

Topic Definition

Synesthesia

Synesthesia is a neurological trait in which stimulation of one sense or cognitive pathway automatically and involuntarily produces an additional experience in another, such as seeing colors when hearing sounds, tasting flavors when reading words, or perceiving letters, numbers and days of the week as having their own colors or locations in space. The experiences are consistent over time, add to rather than replace normal perception, and usually begin in early childhood, although they can also be acquired later through drugs, blindness or brain injury. Synesthesia runs in families, affects roughly 4 percent of people, and is not considered a disorder because it rarely interferes with daily life.

Overview

Synesthesia (also spelled synesthesia) is a neurological trait in which stimulation of one sense or cognitive pathway leads to an automatic, involuntary experience in a second sense or pathway. The word comes from the Greek syn, meaning together, and aisthesis, meaning sensation. A person with synesthesia might see colors when listening to music, taste a flavor when hearing a particular word, or perceive each letter of the alphabet as having its own color. People who have these experiences are known as synesthetes.

The added experience does not replace normal perception - it is layered on top of it. A synesthete who sees the letter A as red still sees the black ink on the page. As neurologist Dr. Richard Cytowic has noted, synesthetic responses are durable, consistent and discrete: the same trigger produces the same experience, often for a lifetime.

Is Synesthesia a Disorder?

Although synesthesia is sometimes called a neurological condition, it is generally not regarded as a disorder because it rarely interferes with daily functioning. Researchers who study it describe the brain differences involved as not problematic and not a sign of disease, and synesthesia has no known effect on overall intelligence [University of Sussex, n.d.]. Many synesthetes say their experiences are neutral or even pleasant, and some report that they were unaware anything was unusual until they discovered that other people did not share them. Others describe feeling as though they had been keeping a secret their entire lives.

Synesthesia is now often discussed as one part of neurodiversity - the natural variation in how human brains are wired and how people perceive the world.

Projecting and Associative Synesthesia

Synesthetes are commonly described as falling into two broad groups:

How Common Is Synesthesia?

Older estimates of how many people have synesthesia varied enormously, from about 1 in 2,000 to 1 in 100,000 or fewer, largely because they relied on people coming forward on their own. A 2006 study published in Perception was the first to screen a general population sample without relying on self-referral and to confirm cases with objective tests. It found that about 4.4 percent of people - roughly 1 in 23 - have some form of synesthesia, making it around 88 times more common than had been assumed [Simner et al., 2006].

The same study found that the most common form was colored days of the week, not grapheme-color synesthesia as previously believed. Grapheme-color synesthesia, where letters and numbers are seen in color, affects about 1 percent of people [Simner et al., 2006]. Around half of all synesthetes have more than one type [University of Sussex, n.d.]. Because many people never realize their perceptions differ from everyone else's, the true number may be higher still.

Early research also suggested that synesthesia was far more common in women than men. Population-based screening has not supported this. The 2006 study found no strong difference between the sexes and suggested the earlier female bias may have come from women being more willing than men to report their experiences [Simner et al., 2006].

Genetics and the Brain

Studies have confirmed that synesthesia is biological, automatic and apparently unlearned, and that it is distinct from both hallucination and metaphor. It runs strongly in families: around 40 percent of synesthetes know of at least one other family member who has it, and it can skip generations [University of Sussex, n.d.].

The precise mode of inheritance is still not known, but genetic research has begun to identify likely mechanisms. A 2018 study in the Proceedings of the National Academy of Sciences analyzed the DNA of three families with several generations of sound-color synesthesia. The rare gene variants differed from family to family, but they were concentrated in genes involved in axonogenesis - the process by which nerve cells grow connections to the right partners - and in cell migration in the developing brain [Tilot et al., 2018]. The findings support the long-standing idea that synesthesia reflects differences in how brain regions are connected.

Behavioral testing, brain imaging and molecular genetics continue to shed light on synesthesia, and researchers hope the work will also help explain how the brain normally organizes perception and thought. More articles on brain function can be found in our Human Brain section.

Acquired Synesthesia

Most synesthesia is present from early childhood, but synesthetic experiences can also appear later in life. Psychedelic drugs such as LSD, psilocybin mushrooms and mescaline can produce temporary synesthesia that fades as the drug wears off, and researchers are not aware of anyone who has acquired lasting synesthesia this way [University of Sussex, n.d.]. Some people have developed synesthesia after becoming progressively blind, although it is unclear whether these experiences are permanent [University of Sussex, n.d.]. Synesthetic experiences have also been reported after a stroke or other brain injury, during temporal lobe epilepsy seizures, and in some people with vision or hearing loss.

This image displays the word SYNESTHESIA and the digits 0 through 9, each character shown in a distinct color, illustrating a common form of synesthesia known as grapheme-color synesthesia.
This image displays the word SYNESTHESIA and the digits 0 through 9, each character shown in a distinct color, illustrating a common form of synesthesia known as grapheme-color synesthesia. In this form, individuals perceive specific letters or numbers as inherently colored, even though they are printed in standard black and white text. For example, the letter S is green, Y is purple, N is blue, and so on, while the numbers are similarly color-coded: 0 appears brown, 1 is bright green, 2 is purple, and 9 is red. Each character also has a subtle glowing or halo effect in its respective color, visually reinforcing the synesthetic experience of associating colors with alphanumeric symbols.

Types of Synesthesia

Synesthesia takes many forms. One catalog of reported variants lists more than 300, although researchers believe these fall into perhaps 5 to 10 broad families, such as language-color, sequence-space and mirror-touch [University of Sussex, n.d.].

In grapheme-color synesthesia, one of the best-studied forms, letters or numbers are perceived as inherently colored. In ordinal linguistic personification, numbers, days of the week and months of the year evoke personalities. In spatial-sequence, or number form, synesthesia, numbers, months and days occupy precise locations in space - for example, 1970 may feel farther away than 1980 - and some people see the year laid out as a three-dimensional map.

A form first described in 2008, visually induced auditory synesthesia, involves hearing sounds such as beeps or taps in response to visual motion and flicker. In the study that identified it, synesthetes were better than non-synesthetes at judging whether two silent sequences of flashing lights matched, because they could hear the rhythm of the flashes [Saenz and Koch, 2008].

Other Types of Synesthesia Include

Cytowic's Diagnostic Criteria

Neurologist Richard Cytowic, one of the researchers who revived scientific interest in synesthesia, identified the following diagnostic criteria:

Living With Synesthesia

Many synesthetes use their experiences to aid their creative work, and synesthetes are more likely than others to take up art as a hobby or career, especially those who see colors in music [University of Sussex, n.d.]. Many also have better memory for a wide range of information [University of Sussex, n.d.]. Non-synesthetes have long tried to capture the experience in art, music and writing.

Synesthesia also appears to be more common in autistic people. A 2013 study in Molecular Autism found synesthesia in 18.9 percent of adults with autism, compared with 7.2 percent of adults without autism - almost three times the rate [Baron-Cohen et al., 2013]. Researchers suggest the two may share differences in how brain connections develop, which could help explain the unusual sensory experiences many autistic people describe.

Synesthesia that has been present since childhood does not need treatment. However, anyone who suddenly develops new cross-sensory experiences in adulthood, particularly alongside headaches, vision changes, seizures or other new symptoms, should speak with a doctor, since acquired synesthesia can sometimes point to an underlying medical cause.

Frequently Asked Questions

At what age does synesthesia first appear?

Synesthesia usually begins in early childhood. The youngest documented case was a three and a half year old boy described in 1922, and color associations tend to become more stable between the ages of 6 and 8 as children learn to read, although they do not yet reach adult levels of consistency.

Can synesthesia fade or disappear over time?

Some people report losing their synesthesia, normally before the age of 10. Some researchers have suggested that all babies may be born with extra connections between the senses that are pruned away in the first few years of life, leaving synesthesia only in those whose connections remain.

Does synesthesia affect intelligence?

Synesthesia has no known effect on IQ. Synesthetes span the full range of intelligence, and while many have stronger memory for certain kinds of information, the trait itself does not make a person more or less intelligent.

How do researchers test whether someone has synesthesia?

The most common method is a consistency test. A person is shown letters, numbers or other triggers on a computer, often more than once and in random order, and picks the color each one evokes. Synesthetes choose the same colors with high reliability, while non-synesthetes tend to choose almost at random.

Do family members always share the same type of synesthesia?

No. Synesthesia itself tends to run in families, but the specific type is not strongly inherited, so one relative may see colored letters while another experiences colored music. Even identical twins can differ, showing that genes are not the whole story.

Can synesthesia cause any everyday difficulties?

Most synesthetes find their experiences neutral or pleasant, but some find them intrusive at times. Some synesthetes also report more trouble with arithmetic, finding their way around or coordination in sports, although these difficulties are not universal.

What is mirror-touch synesthesia?

Mirror-touch synesthesia is when a person feels a touch on their own body while watching someone else being touched. In a 2013 French survey of more than 1,000 people, about 10 percent reported mirror-touch experiences, although these self-reports were not confirmed with objective tests.

Does synesthesia need to be treated?

Lifelong synesthesia is not an illness and does not need treatment. A doctor should be consulted, however, if new cross-sensory experiences appear suddenly in adulthood, particularly with headaches, vision changes or seizures, as this can sometimes signal an underlying medical problem.

References

More on this topic: The Human Brain Publications - Neurodiversity Publications - Autism Publications

Insights, Analysis, and Developments

Editorial Note: Synesthesia is a useful reminder that perception is not the same for everyone, and that differences in how the brain is wired are not always problems to be fixed. For most synesthetes the extra layer of color, sound or taste is simply part of how they have always experienced the world, often bringing better memory and a pull toward art and music. What has changed is the science: once treated as a rare curiosity reported mainly by women, it is now understood as a common, inherited trait with measurable genetic and brain-connection roots, and one that shows up far more often in autistic adults. That shift matters for anyone who has quietly wondered whether their senses work differently, and it also means that a sudden new sensory crossover later in life deserves medical attention rather than being brushed aside.

Ian C. Langtree Author Credentials: Ian C. Langtree is a writer, editor, and the founder and Editor-in-Chief of Disabled World, which he established in 2004 as one of the longest-running independent resources for disability news and information. Over more than two decades, he has written and edited thousands of articles on disability rights, health, accessibility, and assistive technology. Drawing on a global perspective shaped by extensive travel and lived experience, Ian is a committed advocate of the Social Model of Disability, the framework developed by disabled activists in the 1970s that focuses on removing societal barriers rather than on individual impairments. His writing is grounded in that framework and in a lasting commitment to disability rights, accessibility, and social inclusion. To learn more about Ian's background, expertise, and accomplishments, visit his .

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