I have music to shape and color synesthesia. From Wikipedia, I gather that from about 1900 to 2000 psychology research did not see synesthesia as worth studying:
Due to the difficulties in assessing and measuring subjective internal experiences, and the rise of behaviorism in psychology, which banished any mention of internal experiences, the study of synesthesia gradually waned during the 1930s. Marks[7] lists 44 papers discussing colored hearing from 1900 to 1940, while in the following 35 years from 1940 to 1975, only 12 papers were published on this topic. Cretien van Campen graphed the number of publications in the period 1780 - 2000 and noticed a revival of synesthesia studies from the 1980s.
Why was that the case?