What behavioral evidence indicates that synesthesia is real, and not just something that people claim to experience?

a. Some people’s associations match the colors of refrigerator magnets they played with in childhood.

b. Most people change their synesthetic associations from one year to the next.

c. People with synesthesia can find a 2 among 5s, or a 6 among 8s, faster than usual if they have different synesthetic colors, and slower if they have the same color.

d. It is easy to teach someone to develop a synesthesia.