Synaesthesia: Fact or Fiction?   Here are some of the "conclusions" and theories regarding synaesthetes and synaesthesia:                        

How many of us are there? Statistics being statistics, estimates range from 1:100,000 all the way down to 1:200!     The vast majority of synaesthetes are female, outnumbering male synaesthetes by 8/1 in the US and 25/1 here in the United Kingdom.  There are more than likely a lot of people who have it but are unaware as to what it is!

Synaesthetes are neurologically normal and are of  average (and very possibly above) intelligence.

Synaesthetes tend to have superior memories. This is unsurprising as synaesthesia provides the synaesthete  with an additional memory cue. For example, I remember a person's taste as well as their looks and name.

Synaesthesia is involuntary. The perceptions are automatic and cannot be stopped.

Synaesthesia does appear to be inherited through the X-chromosome - via the female line.

Synaesthetes are more likely to be left-handed or even ambidextrous rather than right-handed.

A synaesthete's perceptions are consistent. If "Mark" tastes of pencil lead now, it always will. Some synaesthetes have reported their perceptions have faded with time, but the majority seem to be consistent.