The Kiki-Bouba effect is a non-arbitrary association between some speech sounds and some abstract shapes. This effect was first studied by German psychologist Wolfgang Köhler in 1929. Köhler showed people two abstract shapes similar to the ones we had posted in our story and asked them which was a ‘Takete’ and which was a ‘Maluma’.
In 2001, V.S. Ramachandran and Edward Hubbard repeated this same experiment using the words "Kiki" and "Bouba". 95% to 98% of their subjects selected the curvy round shape as "Bouba" and the spiky one as "Kiki. Research suggests that this effect happens because when you say “Bouba”, your mouth makes a more rounded shape, whereas when you say “Kiki”, your mouth makes a more angular shape.
The shape of Kiki has a sharp inflection, and the sound 'kiki' represented in your auditory cortex (brain's hearing centers), also has a sharp sudden inflection, whereas with 'Bouba' it's more curvy and round. It was also proposed that this Kiki-Bouba effect could be due to the cognitive processes similar to those underlying synesthesia, a condition in which a person experiences sensation in a certain way (e.g. hearing) while a different modality is activated (e.g. seeing a certain colour). This effect is noticed even before language development among toddlers.
However, there is no definite explanation as to why this effect occurs and why there are individual differences.
Peiffer-Smadja, N., & Cohen, L. (2019). The cerebral bases of the bouba-Kiki effect. NeuroImage, 186, 679-689. https://doi.org/10.1016/j.neuroimage.2018.11.033