Synesthesia research

Women: in the U.S., studies show that three times as many women as men have synesthesia; in the U.K., eight times as many women have been reported to have it. The reason for this difference is not known.

Left-handed: synesthetes are more likely to be left-handed than the general population.

Neurologically normal: synesthetes are of normal (or possibly above average) intelligence, and standard neurological exams are normal.

In the same family: synesthesia appears to be inherited in some fashion; it seems to be a dominant trait and it may be on the X-chromosome

Statistics on the exact number of people who have synesthesia run from 1 in 25 to 1 in 100,000. The kind of synesthesia where people associate numbers and letters with color is more common than synesthesia where sound and taste are associated with color.

The exact definition according to the Merriam-Webster dictionary is: 1: a concomitant sensation; especially : a subjective sensation or image of a sense (as of color) other than the one (as of sound) being stimulated2: the condition marked by the experience of such sensations

I myself do not have true synesthesia although I do have synesthete experiences. For example I swear that "L" is a naples yellow.

Information from wikipedia and http://faculty.washington.edu/chudler/syne.html

An interesting website with a personal account of the author's experience with synesthesia as well as a program that allows the viewer to get better insight into what it is like to have synesthesia :
http://www.otherthings.com/uw/syn/