| Summary: | I present new evidence about the relationships between learning and synaesthesia, particularly grapheme-colour synaesthesia, in which individuals experience letters and numbers as coloured. As part of the largest survey of synaesthetic tendencies ever performed, I show that second language acquisition can act as a trigger for the development of synaesthesia, such that children who learn a second language in grade school are three times more likely to develop synaesthesia as native bilinguals. I also demonstrate that previous reports of a sex bias in synaesthesia are almost certainly due to response and compliance biases, rather than any real differences in the prevalence of synaesthesia between men and women. In a detailed examination of the influences of learning on synaesthetic experiences, I show that synaesthetic colours are influenced by knowledge about letters’ shapes, frequencies, alphabetical order, phonology, and categorical qualities. Finally, I demonstrate that synaesthesia can itself be exploited in learning. All these results are presented as supporting a developmental learning hypothesis of synaesthesia, in which synaesthesia develops, at least in part, because it is useful.
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