The bouba/kiki effect is a non-arbitrary mapping between speech sounds and the visual shape of objects. It was first documented by Wolfgang Köhler in 1929 using nonsense words. The effect has been observed in American university students, Tamil speakers in India, young children, and infants, and has also been shown to occur with familiar names. It is absent in individuals who are congenitally blind and reduced in autistic individuals. The effect was investigated using fMRI in 2018. The bouba/kiki effect is one form of sound symbolism.
This picture is used as a test to demonstrate that people may not attach sounds to shapes arbitrarily: American college undergraduates and Tamil speakers in India called the shape on the left “kiki” and the one on the right “bouba”.