This excerpt from The Accidental Mind by David J. Linden provides some interesting statistics on synesthesia:
Synesthetes have normal-to-above-normal intelligence and they appear typical in personality tests and general neurological exams. They do not hallucinate or show an unusual incidence of mental illness. Determining the number of synesthetes in the general population is difficult, but recent estimates have been as high as 1 in 200 people. Synesthesia is much more common in women and left-handed people. Although it is hard to exclude sampling bias, it appears, not surprisingly, that synesthetes tend to be drawn to the creative professions, such as writing, visual art, music, and architecture (p. 90).Although Linden writes that synesthesia occurs in 1 out of 200 people, Cytowic states in his podcast "Wednesday is Indigo Blue: Discovering the Brain of Synesthesia" that at least some degree of synesthesia is actually quite common, occurring 1 out of 20 people! Determining the number of synesthetes is difficult because synesthetes are often embarrassed to share their abnormal ability, or they simply don't realize that what they experience is unique, and may exclaim in astonishment something along the lines of, "You mean the key of G isn't blue to you?!"
A hypothesis to explain synesthesia rests on prenatal pruning. When we are born, our brain is completely connected and the senses are wired together. Yet as we develop, certain connections are eliminated while others are retained. The hypothesis is that when connections are retained that normally shouldn't be (for example, from visual areas to auditory areas), the result is synesthesia. So perhaps we were all synesthetes once! Ramachandran proposes that we still are all slightly synesthetic, with his example of "kiki" and "buba" in his TED talk: VS Ramachandran on your mind.
Now to the good stuff: how does this have to do with creativity? Well, if you think about it, creative thinking involves putting together two seemingly unrelated concepts, such as in the use of metaphor. Alan Rothenberg, prominent researcher of creative processes, argues that janusian thinking, "bringing two opposites together in your mind, holding them there together at the same time, considering their relationships, similarities, pros and cons, and interplay, then creating something new and useful" is a crucial part of the creative process. If this be the case, then wouldn't abnormal connections in the brain that link together "unrelated" areas enhance creativity? More research on synesthesia and creativity to come!