Assume one cartoon character is always happy and another usually not.
Which character is happy, Koko or Kiki?
If you said, "Kiki", "i" probably influenced you.
Ground-breaking experiments have been conduced to uncover the links between language and emotions. Researchers were able to demonstrate that the articulation of vowels systematically influences our feelings and vice versa. The authors concluded that it would seem that language users learn that the articulation of 'i' sounds is associated with positive feelings and thus make use of corresponding words to describe positive circumstances. The opposite applies to the use of 'o' sounds.
The specific focus of the project was on two special cases; the sound of the long 'i' vowel and that of the long, closed 'o' vowel.
In the first experiment, the researchers exposed test subjects to film clips designed to put them in a positive or a negative mood and then asked them to make up ten artificial words themselves and to speak these out loud. They found that the artificial words contained significantly more 'i's than 'o's when the test subjects were in a positive mood. When in a negative mood, however, the test subjects formulated more 'words' with 'o's.
The second experiment was used to determine whether the different emotional quality of the two vowels can be traced back to the movements of the facial muscles associated with their articulation. [emphasis mine]
Researchers instructed their test subjects to view cartoons while holding a pen in their mouth in such a way that either the zygomaticus major muscle (which is used when laughing and smiling) or its antagonist, the orbicularis oris muscle, was contracted.
Rummer and Grice now have an explanation for a much-discussed phenomenon. The tendency for 'i' sounds to occur in positively charged words (such as 'like') and for 'o' sounds to occur in negatively charged words (such as 'alone') in many languages appears to be linked to the corresponding use of facial muscles in the articulation of vowels on the one hand and the expression of emotion on the other. [emphasis mine]
That might have something to do with our tendency to use affectionate nicknames with "-y", and terms of endearment like "Honey" or "Sweetie"!
(Yes, the article was about the English long "i" sound, but that's actually a diphthong that ends in "ee", which corresponds to the laughing/smiling mouth shape.)