Bouba/kiki effect

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 individuals with autism. The effect has recently been investigated using fMRI.
Research.
Discovery.
The bouba/kiki effect was first observed by German American psychologist Wolfgang Köhler in 1929. In psychological experiments first conducted on the island of Tenerife (where the primary language is Spanish), Köhler showed forms similar to those shown at the right and asked participants which shape was called "takete" and which was called "baluba" ("maluma" in the 1947 version). Although not explicitly stated, Köhler implies that there was a strong preference to pair the jagged shape with "takete" and the rounded shape with "baluba".
Extension to other contexts.
In 2001, Vilayanur S. Ramachandran and Edward Hubbard repeated Köhler's experiment using the words "kiki" and "bouba" and asked American college undergraduates and Tamil speakers in India "Which of these shapes is bouba and which is kiki?" In both groups, 95% to 98% selected the curvy shape as "bouba" and the jagged one as "kiki", suggesting that the human brain somehow attaches abstract meanings to the shapes and sounds in a consistent way.
Daphne Maurer and colleagues showed that even children as young as 2 years old may show this effect. More recent work by Ozturk and colleagues (2013) showed that even 4-month-old infants have the same sound–shape mapping biases as adults and toddlers. Infants are able to differentiate between congruent trials (pairing an angular shape with 'kiki' or a curvy shape with 'bubu') and incongruent trials (pairing a curvy shape with 'kiki' or an angular shape with 'bubu'). Infants looked longer at incongruent pairings than at congruent pairings. Infants' mapping was based on the combination of consonants and vowels in the words and neither consonants nor vowels alone sufficed for mapping. These results suggest that some sound–shape mappings precede language learning, and may in fact aid in language learning by establishing a basis for matching labels to referents and narrowing the hypothesis space for young infants. Adults in this study, like infants, used a combination of consonant and vowel information to match the labels they heard with the shapes they saw. However, this was not the only strategy that was available to them. Adults, unlike infants, were also able to use consonant information alone and vowel information alone to match the labels to the shapes, albeit less frequently than the consonant–vowel combination. When vowels and consonants were put in conflict, adults used consonants more often than vowels. 
The effect has also been shown to emerge when the words to be paired are existing first names, suggesting that some familiarity with the linguistic stimuli does not eliminate the effect. A study showed that individuals will pair names such as "Molly" with round silhouettes, and names such as "Kate" with sharp silhouettes. Moreover, individuals will associate different personality traits with either group of names ("e.g.", easygoingness with "round names"; determination with "sharp names"). This may hint at a role of abstract concepts in the effect.
Contexts where the effect is smaller or absent.
Other research suggests that this effect does not occur in all communities, and it appears that the effect breaks if the sounds do not make licit words in the language. The bouba/kiki effect seems to be dependent on a long sensitive period, with high visual capacities in childhood being necessary for its typical development. In contrast to typically sighted individuals, congenitally blind individuals have been reported not to show a systematic bouba/kiki effect for touched shapes. Autistic individuals do not show as strong a preference. Individuals without autism agree with the standard result 88% of the time, while individuals with autism agree only 56% of the time.
Neuroscience.
In 2019, researchers published the first study using fMRI to explore the bouba/kiki effect. They found that prefrontal activation is stronger to mismatching (bouba with spiky shape) than to matching (bouba with round shape) stimuli. Interestingly, they also found that sound–shape matching also influences activations in the auditory and visual cortices, suggesting an effect of matching at an early stage in sensory processing.
Implications for understanding language.
Ramachandran and Hubbard suggest that the kiki/bouba effect has implications for the evolution of language, because it suggests that the naming of objects is not completely arbitrary. The rounded shape may most commonly be named "bouba" because the mouth makes a more rounded shape to produce that sound while a more taut, angular mouth shape is needed to make the sounds in "kiki". Alternatively, the distinction may be between coronal or dorsal consonants like /k/ and labial consonants like /b/. Additionally, it was shown that it is not only different consonants (e.g., voiceless versus voiced) and different vowel qualities (e.g., /a/ versus /i/) that play a role in the effect, but also vowel quantity (long versus short vowels). In one study, participants rated words containing long vowels to refer to longer objects and short vowels to short objects. The presence of these "synesthesia-like mappings" suggest that this effect might be the neurological basis for sound symbolism, in which sounds are non-arbitrarily mapped to objects and events in the world. Research has also indicated that the effect may be a case of ideasthesia, a phenomenon in which activations of concepts (inducers) evoke perception-like experiences (concurrents). The name comes from the Greek "idea" and "aisthesis", meaning "sensing concepts" or "sensing ideas", and was introduced by Danko Nikolić.
In media.
The experiment was reproduced in episode 3 of season 4 of the television show "Brain Games" with the names "takete" and "maluma." One participant expressed her association of takete and maluma with their respective shapes by comparing them to the rigid movement of a toy soldier and the swaying of the hula, respectively. A similar experiment included the association of the name "lomba" with a fictitious brand of milk chocolate and "kitiki" with a fictitious brand of dark chocolate.