How rare is synesthesia? I visited a neuropsychologist recently who told me I have Synesthesia. I never really knew what it was, but it's fascinating and I can't believe it isn't the norm among people. For me, certain numbers just have had personalities.

For some reason, the more difficulty I had with the number, the more complex personality it had. For instance, I had a lot of trouble with odd numbers, and larger numbers. Therefore, 1 and 2 do not have personalities. On the contrary, 7 and 9 are the most developed personalities. 

How rare is this condition?
From Hubbard &amp; Ramachandran (2005):


  [...] the estimated prevalence of synesthesia has varied dramatically, between as many as 1 in 20 (Galton, 1883) and as few as 1 in 25,000 (Cytowic, 1989). The most widely cited study to date suggests that synesthesia occurs in at least 1 in 2000 people (Baron-Cohen et al., 1996), although this is now generally regarded as an underestimate. Our own informal samples have suggested that the prevalence of grapheme-color synesthesia might be between 1 in 200 (Ramachandran and Hubbard, 2001b) and 1 in 100 (Mulvenna et al., 2004). Subsequent large-scale studies have suggested that the prevalence of synesthesia might be as high as 1 in 20 across all forms and 1 in 100 for grapheme-color synesthesia (J. Simner, personal communication). This high prevalence argues against the notion that synesthesia is merely a “benign cognitive variant” (see Ward and Mattingley, 2005) and instead suggests that it is a widespread phenomenon that may provide novel insights into the neural basis of the mind (Ramachandran and Hubbard, 2001b).


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References


  Hubbard, E. M., &amp; Ramachandran, V. S. (2005). Neurocognitive mechanisms of synesthesia. Neuron, 48(3), 509.