"The cause of synesthesia is still subject to research, but it’s generally believed to be the result of a genetic mutation on the X chromosome, explaining its dominance in woman and high heritability. Some researchers think its heritability could suggest an evolutionary benefit. Sickle cell anemia, for example, can be deadly, but also provides malaria immunity. Does synesthesia provide a similar benefit?

It might if you’re a mathmetician or an artist. One of the peculiarities of some forms of synesthesia is that equations are visualised in 3D space, which might help someone like physicist Richard Feynmann, another famous synesthete, with his work. David Hockney, also a synesthete, once told Robert Burton that when he was designing a piece of art intended to accompany a production of a Maurice Ravel piece, he listened to the relevant section of the score and “the tree painted itself.” It’s also been suggested that savants like Daniel Tammett get their incredible skills from…"