Synesthesia

How it works

Synesthesia is a condition in which input to one sense or concept automatically produces an experience in another¹. The most studied form is grapheme-colour synesthesia. A person with it sees or strongly associates a colour with each letter and number. The letter A might always be red and the number 7 always green. The pairings are stable. Tested months or even years apart, the person gives nearly the same answers¹.

Other common forms link sounds to colours, or sequences such as months and numbers to positions in space. More than 70 forms have been described¹. Some people with synesthesia have more than one form. The most commonly reported type is coloured days of the week¹.

Synesthesia runs in families. In one study of 53 synesthetes, 42% reported a parent, sibling or child who also had it⁴. Several genes have been linked to it, but there is no single synesthesia gene. One explanation is extra connections between neighbouring brain areas, for example between the area that recognises letters and the area next to it that handles colour¹. One idea is that these connections exist in all babies and are normally pruned away.

Synesthesia is not a disorder. Most people with it consider it normal or pleasant, and some do not know it is unusual until they learn that other people do not have it¹.

How common it is

The 4.4% figure comes from a 2006 study that tested people directly for consistent pairings, rather than relying on people to come forward, which is what made earlier, much lower estimates too low¹. A test-based count like this is stronger evidence than a self-report survey, because it checks whether the same person gives the same answer months apart. Rates for a single specific type, such as grapheme-colour, are lower than the all-types figure.

Is it a problem?

No. Synesthesia is not a disorder and needs no treatment¹. It does not affect intelligence or general health, and most people who have it find it neutral or enjoyable. Mention it to a doctor only if paired sensations appear suddenly for the first time in adulthood, or come with other new neurological symptoms, since lifelong synesthesia and a sudden new pairing have very different causes.

History and culture

The novelist Vladimir Nabokov described his coloured alphabet in detail in his memoir. His mother and his son had it too³. The composer Olivier Messiaen and the painter Wassily Kandinsky are often described as synesthetes¹. Online communities such as r/Synesthesia let people compare their colour alphabets². Two people’s full alphabets usually differ, though some trends are shared, such as A often being red¹.