Although I may sound bitter in my personal opinion of synesthetic research to date, I have reason to do so. Ever since I was a kid I was (luckily) understood to be syne and supported. Realizing to be able to achieve basically anything very well dramatically reverted the initial confusion into something appreciated and I grew up sorting out what is comprehended by others and what is not, easing natural social interactions. Being capable of mingling among others has never meant actually seeking it, and as many other synes I personally tend to be withdrawn. Speed of reasoning usually clashes with one's knowledge pool too, both ways: this causes a sensation in the syne individual that ranges from inadequacy all the way to arrogance, which socially tends to label a person with one or two major recognizable and commonly understood characteristics. An influencible synesthete may learn to believe that what others pin him or her to be must be correct - but I am 100% certain that 100% of the times it is 100% not: it's simpler to understand that the synesthete finds it too complicated, tiring or aggravating to try and explain who he or she really is. However, this digs into human personality aspects and it goes way off topic.
Focusing on what syne research usually defines as 'correct', the individuals are classified with a numeric form of 'fold' measuring system. Along with this, some set characteristic are acceptable, such as seeing numbers and letters in differing colours for example, and many tests are run under these set rules. But there is only one rule about synesthesia, and that is that there are no rules. Anything that has any defined boundary cannot coexist with synesthesia. Again, an influencible syne individual may force himself to believe he should 'fit' into these set rules and accomodate his natural sensations as much as possible to be 'part of a group', which again digs into simple human nature and wanders off topic.