Number form synaesthesia ~12% (Ward, Sagiv & Butterworth 2009)
Dyslexia 5-10% English-speakers (Mitchell Feb 2011)
Dyscalculia 5-6% (Mitchell Feb 2011)
Congenital amusia (tone deafness) 4% (Mitchell Jan 2011) (Mitchell Feb 2011)
Day of the week -> colour synaesthesia 2.8% (Banissy et al 2009)
Prosopagnosia 1-2% (Mitchell Feb 2011)
Congenital prosopagnosia 2.5% (Mitchell Jan 2100) (This figure is inconsistent with the above figure as people with congenital prosopagnosia should be a sub-set of all people who have prosopagnosia)
Mirror-touch synaesthesia 1.6% (Banissy et al 2009)
Grapheme -> colour synaesthesia 1.4% (Banissy et al 2009)
ASD including autism ~0.6% (Wikipedia)
So this means that, if the disorders besides autism listed above do not overlap in the people they affect, possibly almost a quarter of the population either can’t read, can’t do maths, can’t comprehend music normally, or can’t recognize faces adequately, while diagnosable autism is thought to only be found in less than a percent of people. So why so much hysteria and research funding about autism and so little funding for research into all the other issues?
The total number of synaesthetes in the population cannot be calculated by simply adding up the different types of synaesthesia listed above, because we know that individual synaesthetes often have a number of different types. Regardless, it is clear that synaesthetes make up a sizeable proportion of the population, and synaesthesia isn’t rare at all. So why is it that most teachers that I have spoken to have never heard of synaesthesia, a neurological condition (not disorder) that can directly affect learning (positively and on occassion negatively) and can affect the student’s sensory experience in the classroom?
Banissy, Michael J, Kadosh, Roi Cohen, Maus, Gerrit W, Walsh, Vincent, Ward, Jamie Prevalence, characteristics and a neurocognitive model of mirror-touch synaesthesia. Experimental Brain Research. (2009) 198:261–272. Published online: 3 May 2009. DOI 10.1007/s00221-009-1810-9 http://www.springerlink.com/content/26mh37152110617x/fulltext.pdf
Mitchell, Kevin The Neuroscience of Tone Deafness: The strange connection between people who can’t sing a tune and people who are “face blind”. Scientific American. January 18th 2011. http://www.scientificamerican.com/article.cfm?id=the-neuroscience-of-tone
Mitchell, K. J. Curiouser and curiouser: genetic disorders of cortical specialization.Current Opinion in Genetics & Development. 2011 Feb 4. [Epub ahead of print] http://www.ncbi.nlm.nih.gov/pubmed/21296568
Ward, Jamie, Sagiv, Noam and Butterworth, Brian The impact of visuo-spatial number forms on simple arithmetic. Cortex. Volume 45 Issue 10 Pages 1261-1265 (November 2009). http://www.cortexjournal.net/article/S0010-9452(09)00213-5/abstract